In the Marvel Comics universe, the Norse gods are real. Thor, Odin, Loki and friends actually exist in that universe. The Earth in the Marvel Universe is pretty similar to the actual Earth, including religions and myths. Logically, the Asgardians had contact with Earth a few centuries ago, which provides the source for the Norse religion in that Earth. My question is about the time between then and the time that Thor comes to Earth in the modern day. Has it been explained why Asgard cut off contact with Earth for all that time? Why'd they interact with the Marvel Earth centuries ago in the first place?
Has it been explained why Asgard cut off contact with Earth for all that time?
According to the Marvel Wiki entry on the Celestials:
The Third Celestial Host arrived on Earth one thousand years ago to inspect the progress of the human race. Their landing site was arranged by the Eternals working in conjunction with the Incas of Peru. The Third Host was met by a contingent of Earth's mythological gods, including Odin of the Asgardians and Zeus of the Olympians, who challenged the Celestials' right to interfere in Earth's affairs. The outcome of that encounter was that all of the major races of gods swore to forego their active involvement in the destiny of mankind.
As for your second question:
Why'd they interact with the Marvel Earth centuries ago in the first place?
According to the entry on the Asgardians:
Inhabiting the Nine Worlds in the other-dimensional Asgardian system are six races of humanoid life forms. Each race is different and intelligent, but the most powerful race is that of the Gods. The Gods are the most human looking and believed to have inhabited Earth at one time only to move to Asgard sometime later.
So presumably in ancient times the Asguardian "gods"
were originally humans who were somehow elevated to immortal status† and emigrated to Asguard. Presumably they still cared about their "homeland" until they were forced to give up active involvement in the destiny of mankind.
† Note: The Eternals Marvel wiki entry specifically references the Olympian pantheon but doesn't mention the Asgardians. The Eternals are an evolutionary offshoot of humanity; they are beings possessing greater powers and longer lifespans than the mainstream human race. They were created over a million years ago by the Celestials.
Second Note: Reading a bit more on the Eternals, it seems that Zuras, an eternal, was chosen to be Zeus' representative on Earth. So the true Olympian "gods" were not originally human, but arose from absorbed energies given life by human dreams. Odin's father, Bor, was "among the gods that created the universe." Thor was the son of Odin and Gaea, who "became the mother god to virtually all of the pantheons of later gods in humanity."
So most of the Marvel universe "gods" had ties to Earth since it was the sentience of Human dreams that originally gave them life.