Realistically speaking, almost the totality of the NFTs market is concentrated on the Ethereum blockchain.
Where did you get this statement? It's concentrated in ethereum blockchain but it supports multiple network. NFT market is using one for all strategy and it's not specifically for ethereum but NFT market is open for any NFT asset that was using various blockchain.
I know that there are some marketplaces on BSC too (Bakery swap for instance), but are almost irrelevant in terms of volume and liquidity if compared with the ones on Ethereum.
It's caused by NFT market that build in ethereum already covered almost all of relevant blockchains. That means people just need to use open sea and that's more than enough to be used to trade their NFT assets. It's supporting multiple blockchain. that doesn't make sense if any blockchain needs to buil their own NFT market. I think that this is the most relevant reason for that.
Do you believe that one day people are going to move on BSC to sell their NFTs?....if not, why, what does make people choose Ethereum?
I don't. Since people are feeling confidence with opensea and they will still stay on it.