Web Standards & Doctypes?

I’ll be the first to admit that I don’t know enough about web standards. I’ve been designing web sites for several years and have known about web standards for only the last few years. In order to learn as much as possible about web standards I now read lots of Web Design blogs. Thankfully I’m learning more and more from those blogs. There’s always plenty of interesting posts on web standards, accessibility, usability, and the likes.

But what I don’t get is why there is the need for so many Doctypes. Can someone please explain it to me?

With all the talk about this site and that site conforming to Web Standards and this site and that site not conforming to web standards, why isn’t there anyone trying to simplify the situation?

Why isn’t there only two doctypes? One for standard compliant sites and one for non-compliant sites. In fact, why isn’t there only one doctype – a standard compliant one?

Why do we need three doctypes just for HTML 4.01, and three more for XHTML 1.0? What’s the rational behind this? I have honestly no idea, but hope that someone will impart their wisdom upon me and make me all the wiser :)