It's not that the premises and protocols themselves are complicated — sure, they're technically sophisticated, but they can be explained simply. The problem lies in that it is very hard to predict the economic consequences and security guarantees of the various consensus mechanisms. Frankly, if Bitcoin hadn't been working flawlessly for the past 12 years very few people would guess that it would work.
The second is that PoS has significantly worse security properties than PoW.
The core reason is that "changing your mind" (rolling back the blockchain) is free with PoS, except for possible loss of value of the cryptocurrency, whereas it's extremely expensive with PoW. There are all sorts of band-aids you can slap on like ignoring block reorgs of a certain depth, but then you lose properties like network partition tolerance.
EDIT: adding source https://beaconcha.in