We use to be real strict about members searching through existing posts to find an answer before starting a whole new thread. But as the forum is so slow lately, it hasn't been enforced for a while. But there are many threads about this very thing on here already.
I'd start with
this thread, especially post number
47 as it was done by Craig McCracken himself. There is also
this thread, started by Lauren Faust herself, and also
this thread, all of which contain info as to why the show came to an end. It wasn't really "cancelled". They just stopped production, really. And it had a good run while it lasted. It lasted much longer than most other shows now days, and for that we are all right proud.