How can it be that some old recordings sound sublime?

When lp records first hit, for many years great care was taken in regards to sound quality because it was a new and marketable thing and the sound is what helped sell as much as the performance.   The technology needed to make good recordings was not an issue even then.  

Over time  as the novelty wore off record product quality became more hit or miss much as it is still today.  

Also as mentioned recordings were all analog mastered largely with tube based gear which imparts a unique sound to recordings of the era preceding transistors. That makes most any recording from the time audio candy of a sort.  

Also so nowadays audiophiles have access to better playback gear than ever so you get the best of both worlds and old recordings that may have never had a chance to shine now can.  Remastered properly to CD and digital even.