Because the Roman Emporer realized Rebbe’s greatness, and asked Rebbe if he will have a Chelek in Olam Haba although he is from the rulers of Esav. Yes, that was a good period under the Roman’s. Rebbi Yehuda Hanasi knew that it won’t last, and he therefore organized the Mishnayos.
Roman trouble started pretty much from when they were introduced to the land. There might have been some better times after they felt the Jews were weak enough and unorganized. That was many generations after the Churban. There were probably barely any Amora’im left in Eretz Yisroel. Once the Bizzantine government took control things got much worse.
This is all a side point, though. Am I safe to assume that the Lag Ba’omer-rebellion connection is a recent invention?