Because much of the code he used was contributed by others. When those others reused that code, he wrote a cease a desist letter to them, saying that they stole the code.
This kind of activity is precisely why the GPL was invented.
Do you just like making stuff up? Altair users were making copies of the paper tape that Micro-Soft and sharing them. Gates, Allen, and Davidoff wrote and original implementation of a unlicensed programming language and offered it for sale.