Rabbi Yehuda HaNasi (Judah the prince) lived where the Supreme Court was, which was Bayt Sharim. The torah shebialpeh was being forgotten, so he had to make a decision whether to write it down or not. If he did, it would break a certain tradition that you are not supposed to write it down. But in this case writing it down was not breaking the tradition. He divided the torah shebalpeh into two categories, which are all the mishniyot zaraeam, moad, nasheam, nazeakym, kidoshym, and tiharot. If you take the first letter from each word it spells zman nakat, which means take time "to learn". He moved from Bait Sharim to Tepori because of his health. In Tepori he taught the Mishna. .
Jewish people have two gamaras, Talmud Yiroshalmi and Talmud Babbli. We learn more Babbli and the question is why? The nasi was the ruler of Israel, like a president. Rome supplied money to the Jews in Israel. All of a sudden, at about 400 C.E. the Romans decided to stop supplying money. Since there was no more money for Israel it was getting smaller and poorer until it was gone. Around 426 C.E. the office of the Supreme Court stopped because people were too poor. Then there was a depression and the Jews had to leave Israel for Babel (Babylon). The amount of rabbis, schools, and scholars increased in Babel. In Israel the population was rapidly declining. The reason the Jews went to Babel was because after the destruction of the first Bait Hamkidash they went to Babel, and that's why they chose Babel. Another reason they went to Babel is because the Tigris and Euphrates rivers were around there, so they could drink and had fertile land. The government was lenient towards the Jews, which made it comfortable for learning Torah. That is why we learn more Babli than Yirushalmi. Resh Glita, the descendent of David was in charge of everything and kept Babel running.