Yes, there's backwards compatibility to maintain, but surely some of it can be contained, maybe with shims, like Windows does it.
In all likelihood, because the people maintaining Wordpress are the people who created it in the first place, and they don't know any better.
Cultures are hard to change. The people that are turned off by Wordpress' programming style probably pick other projects and other communities. Lots of the folks working on Wordpress probably cut their teeth in that codebase. They might not like it and they might realize there are better ways, but there probably isn't a critical mass of them that share a vision out of the morass.
Personally, I took one brief look under the covers of Wordpress and immediately decided to avoid it.