You asked: Why was Urantia (earth) placed under quarantine, and by who?
This is an important question...a question whose answer can explain a lot of the turmoil in our world.
In the medical world, when a patient has contracted an infectious disease, that patient is often isolated from healthy people, lest these healthier ones catch the disease. An epidemic could develop if the sick patient is not quarantined for the duration of the illness.
Long, long ago, a spiritual sickness fell upon our earth, Urantia. It was a spiritual sickness whose symptoms were a denial of God, a false celebration of personal liberty, and an attack against the Creator of our local universe, who eventually incarnated as Jesus of Nazareth.
This spiritual sickness was brought about by the collusion of our planetary prince, Caligastia, with Lucifer in his destructive rebellion against God. The details of Lucifer's blasphemous ideas can be read in his "manifesto" HERE. And the complete details of the rebellion can be read HERE
From The Urantia Book:
67:0.1 THE PROBLEMS associated with human existence on Urantia are impossible of understanding without a knowledge of certain great epochs of the past, notably the occurrence and consequences of the planetary rebellion. Although this upheaval did not seriously interfere with the progress of organic evolution, it did markedly modify the course of social evolution and of spiritual development. The entire superphysical history of the planet was profoundly influenced by this devastating calamity.
67:3.1 The outbreak of rebellion on Jerusem, the capital of Satania, was broadcast by the Melchizedek council. The emergency Melchizedeks were immediately dispatched to Jerusem, and Gabriel volunteered to act as the representative of the Creator Son, whose authority had been challenged. With this broadcast of the fact of rebellion in Satania the system was isolated, quarantined, from her sister systems. There was "war in heaven," the headquarters of Satania, and it spread to every planet in the local system.
Click to read more about The Planetary Rebellion
When our Creator Son, Christ Michael, was incarnated on our world as the humble babe of Bethlehem, one of his life missions was to end the rebellion -to reclaim this world from the rebels and assume final sovereignty over it. And this was accomplished during Jesus' life on a fateful day on Mt Hermon when Jesus was 31 years-old.
Because of the planetary quarantine, our planet still suffers in relative spiritual darkness. Many of the ideas of the Lucifer rebellion are still alive and well here; ideas such as atheism, greed, and selfishness. The quarantine means that we are a very disordered world. One might wonder why these things are permitted if it is true that Jesus forever settled the rebellion. The answer is that even though the rebellion is over, the quarantine is not.
On a normal planet – a planet on which rebellion has never occurred – the inhabitants of that planet enjoy a regular communication with celestial sources; such a planet is included in the great spiritual circuitry of the universe. In addition, a normal planet benefits from the presence of an age-old civilization of spiritual origin (our Dalamatia would have been such a civilization, had it not been obliterated by rebellion), and a younger, though equally as progressive civilization which is established by a Material Son and Daughter (like our Adam and Eve) whose Garden of Eden endures generation after generation – a testament to the Father. Our Adam and Eve defaulted in their mission and are no longer with us, as they should be. This unfortunate default was a direct casualty of the rebellion, too. People on normal planets do not wonder about the existence of God. They are all familiar with the Garden of Eden, and are accustomed to spiritual contact. Here on Urantia, we grope in spiritual darkness.
Why are we still under quarantine?
54:5.10 On Jerusem the personal representative of the Supreme Executive of Orvonton counseled Gabriel to foster full opportunity for every living creature to mature a deliberate choice in those matters involved in the Lucifer Declaration of Liberty. The issues of rebellion having been raised, the Paradise emergency adviser of Gabriel portrayed that, if such full and free opportunity were not given all Norlatiadek creatures, then would the Paradise quarantine against all such possible halfhearted or doubt-stricken creatures be extended in self-protection against the entire constellation. To keep open the Paradise doors of ascension to the beings of Norlatiadek, it was necessary to provide for the full development of rebellion and to insure the complete determination of attitude on the part of all beings in any way concerned therewith.
This development is still in progress. Lucifer and Satan are detained, and awaiting adjudication. In the celestial spheres, time is reckoned differently than here. For them, the passing of several thousand years is not realized as it is here. It likely seems a fairly short interval between the time of rebellion and the adjudication for them; for us, the time seems interminable. Meanwhile, we suffer under quarantine.
46:8.3 But ere long, the adjudication of Lucifer and his associates will restore the Satania system to the Norlatiadek constellation, and subsequently, Urantia and the other isolated spheres will be restored to the Satania circuits, and again will such worlds enjoy the privileges of interplanetary communication and intersystem communion.
That will be a happy day! But, it may be awhile:
54:5.12 An emergency council of ex-mortals consisting of Mighty Messengers, glorified mortals who had had personal experience with like situations, together with their colleagues, was organized on Jerusem. They advised Gabriel that at least three times the number of beings would be led astray if arbitrary or summary methods of suppression were attempted. The entire Uversa corps of counselors concurred in advising Gabriel to permit the rebellion to take its full and natural course, even if it should require a million years to wind up the consequences
But, however difficult life may be here as a result of rebellion and quarantine, we are given solid hope for our future potential in these passages:
50:7.1 On first thought it might appear that Urantia and its associated isolated worlds are most unfortunate in being deprived of the beneficent presence and influence of such superhuman personalities as a Planetary Prince and a Material Son and Daughter. But isolation of these spheres affords their races a unique opportunity for the exercise of faith and for the development of a peculiar quality of confidence in cosmic reliability which is not dependent on sight or any other material consideration. It may turn out, eventually, that mortal creatures hailing from the worlds quarantined in consequence of rebellion are extremely fortunate. We have discovered that such ascenders are very early intrusted with numerous special assignments to cosmic undertakings where unquestioned faith and sublime confidence are essential to achievement.
54:6.4 But one thing should be made clear: If you are made to suffer the evil consequences of the sin of some member of your family, some fellow citizen or fellow mortal, even rebellion in the system or elsewhere—no matter what you may have to endure because of the wrongdoing of your associates, fellows, or superiors—you may rest secure in the eternal assurance that such tribulations are transient afflictions. None of these fraternal consequences of misbehavior in the group can ever jeopardize your eternal prospects or in the least degree deprive you of your divine right of Paradise ascension and God attainment.
A really good question...I hope this reply is helpful to you!