Compression or recording and mastering engineer's that do not know what they are doing.
Why do some cd's have no bass
I usually listen to roots music and I just picked up Marty Stuarts new cd and the bass is way,way back in the mix. The rest of the instruments sound great. Its a very good rockabilly recording with great tunes , just no bass. I thought that style of recording went away with Buck Owens who recorded that way so the tunes would stick out on AM radio.