Is remastered mainly just less jitter?

.... less jitter?

Who knows. The re-clocking with the laser burners slows down the production rate significantly and the result is, what nobody wants: higher production costs.
There are so many ways to make a CD worse (or the master) there are endless reasons  to make a reissue. But the main reason for me is to make money again and again.
I seriously think (with a few exceptions) when you want the most natural sound on a CD, look for one of the first releases, beginning of 90's, those are pretty good.

I listen to Classical exclusively.  The remastering are almost always superior.  There are none of the loudness issues.  From what I have read the biggest reason for the improvement is that early CD issues of analog recordings featured 14 bit ADCs.  Also several Conductors liked to diddle with the knobs in the control booth and did a lot of damage, so going back to the earlier mixes was restorative in and of  itself

Of course Classical and Jazz will do better, rock is a garbage in, garbage out situation .

Young folks boogie on headphones and iPods. Old dudes listen to classical on their big home systems. The market is young people. Which is why rock and pop are compressed and classical isn't.

Market is not for young people its for dumb people which come in all ages, esp. in USA .