I know there are others who can do a better job of explaining the origins of the Baptist denomination, but for my personal observation I would say that, unlike with most other denominations, the Baptist denomination exists by not establishing doctrine that would run into conflict with most other denominations. If I'm not mistaken, I think that the term, fundamentalism, was coined by Baptists, and it pretty much explains everything about their continued existence. The tenets of this group, similar to what you might hear being preached on any given Sunday, are usually such that no good Christian could disagree. It leaves a lot of room for debate about all remaining issues, and I find that this denomination, refreshingly, actually has a lot of healthy debate (some would say unhealthy, but I strongly disagree, and not just because I like the irony of debating about debate). It might explain why in my church only a small minority have a Baptist background, despite being a Baptist church. It might also explain why a higher percentage of the sermons stay on core issues, like salvation.
I prefer to have several denominations available, even if they all had exactly the same beliefs, if for no other reason than to keep another Catholic Church from forming among us.