There are a lot of dialects because its core is one of the easiest programming languages to understand and implement, and once implemented, it's one of the easiest languages to extend. That's all there is to it, pretty much.

Anyone who is interested in the design of programming languages will find it easy to make their own Lisp and try out any ideas that seem congenial to them. The natural and obvious consequence is a lot of Lisps.

Creatives gotta create.