Why did earlier leaders like the Avos, shevatim, Moses etc. are not called RABBI and are just referred to by first name unlike later generations from about the time of the Mishnah.

Different generations express honor in various ways. At the beginning, there were no titles, at all. Then came Rabbi, Rabban, Rav in various locales, at different times. In short, we always follow the local custom, and continue referring to each rabbi as they did in his respective period.