Why are there Baptists, and American Baptists, and Southern Baptists, and Free Baptists, and Conservative Baptists and a bunch of other kinds of Baptists?
Well, as to Southern Baptists and American Baptists, there was this little problem sometimes known as the War of Northern Aggression. The division today is more of a liberal/conservative issue.
There is a lot of theological diversity among Baptists today, as well.
And finally, there is the saying that if you have two Baptists, you have a church. If you have 3 Baptists, you have 2 churches. It isn't really that bad, but since it is a voluntary association rather than a centralized denomination, no one can really tell a local church what to do or to believe.