How can it be that some old recordings sound sublime?

Modern live recordings aside, you're talking about two very different things.  Once you get very far beyond two mikes and a live recording you're putting together pieces of a puzzle rather than taking a snapshot of an actual event.  Non-live with multiple overdubs makes for a tricky process that is not likely to result in a natural sound.  There's also a difference between recording/reproducing acoustic versus electric sounds.  I, too, am often surprised if not astonished at the great sound of some recordings from the fifties, mono and stereo alike.

The reason they used a simply recording techniques is because they didn't have any alternatives.  4 or 8 track recorders weren't in general use, so everything had to fit on two channels.  As a result engineers used a small number of microphones and ran them through very basic mixers.  The only sound processors available were EQ and compressor/limiters which were used across the 2 channel mix.  A decade later you would have 16-24 channels with individual EQ on each channel.  The drums could have 4 mics, the piano 2 mics and separate mics for the bassist and each horn plus dedicated "room" mics for crowd noise.  For this you needed a bigger, more complicated mixer with multiple gain stages.

German tube condenser mics and RCA ribbons were the microphones of choice back then and they are superb instruments that are highly valued in today's market.  But engineers from that time period thought they were finicky.  Their sound could change from session to session or even within a 3-hour session.  Transistor mics quickly replaced the better sounding tube mics because they didn't have this problem.

I concur w/ celtic66-

Agree with all of the above. Great tube equipment with people who cared about the sound. There is something very transparent about many of these recordings both classical and jazz. Like the Nelson riddle Capitol recordings from the late 50s. Fritz Reiner bartok recording from the mid fifties. But there are also great recordings being made now. The most recent Justin beiber record "purpose" is very beautiful sounding and I have a several great orchestral recordings of current composers music that sound transparent and beautiful. 

The point of the whole thing was different, too.  Getting a good reproduction of what was being played used to be a priority.  Getting the listeners' attention in a crowd seems to be more important now.  Different equipment, different approach, different priorities.