I think putting aside the legal issues with most countries,  there is also the problem of transaction confirmation,  price fluctuations and trust in transaction, though this can be minor reasons compare to the legal issues, many countries still have restrictions to use or accept btc, this can be challenging to most small and medium sellers,
Again, like you said, I think people still rely heavily on the use of fiat card payment than using btc, it is more in theory than in practice, I think haven't gotten use to using it yet, and there are not much places to use it.