My guess is that people don't update because they fear potentially breaking their styles or something.
That said, I don't use any plugins, so I didn't have to worry about compatibility issues.
The upgrade process may be simplified but who wants to spend hours trying to fix compatibility every time they discover a bug they introduced with the last patch because they've broken backwards compatibility yet again.
What they should be doing is a general upgrade release for non-technical users and a technical bulletin about what exactly was wrong so technical users can manage older versions as desired. Instead you have to dig for an hour in trac notes to find what the heck they changed and why.
I have version 2.3 & 2.5 installs running safe because I've manually patched them and locked down the server. Delete any XMLRPC interface which is where half the bugs are introduced. The other half is the open ended admin interface which even regular users are allowed into to escalate privileges, which is asinine - you can even run PHPINFO through the admin panel as a regular member on many WP installs.
I immediately got the "white screen of death" afterwards and had to do a binary search to figure out which plugin(s) were the culprit (and then try to track down newer versions or alternatives).
Some API changes are not backwards-compatible, so your themes have to be upgraded also (for example, the "show comments" api will by default say "comments off" on every page that doesn't allow them, which it didn't before). I'm sure there's other such changes I haven't yet noticed.
I'm glad I upgraded, but it was a major hassle and nearly a full day's work in my case.