The Talmud Yerushalmi is written in Aramaic because that’s what everyone in Israel spoke at the time. Aramaic was the lingua franca of the Middle East at the time, before it was replaced by Arabic. The Palestinian dialect was different than the Babylonian dialect because that’s what happens when people live far away from each other (Like British English vs American English vs Australian English). It is also harder to study because before it could be finished, everyone moved to Bavel (which is why we hold like the Bavli in case of a machlokes)
