Why are reissued jazz/soul/funk/etc. CDs mastered at a lower volume level than new releases?

I'm not making this up, it's the case with *every* reissue I've bought, regardless of the label.

Trying to accurately recreate the albums as they sounded when released - instead of compressing the shit out of them.

Huh? I'm not familiar with mastering techniques... Why would rising the volume level change the sound?

If you want old jazz records to appear as "loud" as newer ones you have to compress them, then you lose some of the dynamics in the music and it sounds more flat.

Rising the volume level when you've already got the music peaking at full-scale (0dB on CD) involves compression or brick-wall limiting - essentially you squash all the transients while raising the averaged-over-time level (which is what the ears perceives as volume). Done subtly over different frequency bands this is not necessarily a bad thing and can sweeten the sound as well as give it a little more punch; done with the severity that is now the norm in pop/rock CD mastering you end up with no dynamics whatsoever and a fatiguing blare.

If it's a new reissue and it sounds relatively quiet (i.e. it isn't merely peaking at -10dB or something as was the case with early CDs), it's probably been remastered with a bit of care.