I don't think development actually stopped. ST3 was in a quiet public beta for a long time, but you can see builds of ST3 from 2013 to 2019: https://www.sublimetext.com/3.

Why would you stop developing for 5 years?

Maybe they just wanted to do something else. Sometimes people just don't want to grind on endlessly for theoretically more money when what they've got already is enough for them.

Probably to write sublime merge