From what I've read, Microsoft BASIC was considered too bloated for a 4K machine, plus Bill Gates wrote an open letter to hobbyists and suggested they were all pirates, so a smaller BASIC was developed called "Tiny BASIC":

"Tiny BASIC was published openly and later invented the term "copyleft" to describe this. This made it popular in the burgeoning early microcomputer market."

https://en.wikipedia.org/wiki/Tiny_BASIC