I looked into the changes between Python 2 and 3 a few years back (like, 2011, 2012). There lots of minor changes that were all nice, but no single big change that was a compelling reason to switch. If, e.g. they had ditched the GIL and allowed pure Python code to exploit multicore systems, that would have been compelling to me.

Instead, at the time, Wikipedia claimed that Python 3 ran somewhat slower on CPython than Python 2. :(

Maybe all of those minor changes taken together constitute a big improvement, once I give it a try. The Unicode handling is said to be much better, and Unicode handling does suck really, really hard on Python 2. So there's that.

Because Python 2 will be phased out in 5 years.