I also accept a point about legal issues...
Legalising crypto's in many countries...will bring big players to participate and when such exposure happens...i think lot's of the technological issues can be sorted out...

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,
Even the volatility of bitcoin alone is enough why these marketplaces is not yet accepting cryptocurrency payments. These companies won't let the decentralized network decide when they can get their revenue not on a coin whose price is dependent of demand and supply laws. Well aside from legal issues, coz there are many imposed against bitcoin, there have been a long talk on bitcoin whether if it should be on the methods or not, so far I've seen some adoption and it's great.
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.
Look, bitcoin will only take a second to transfer but fiat will take you less time than that, imagine you are just buying a clothe, it's easier to pull out your wallet and get a couple of bucks than opening your phone, go to e-wallet, scan the merchant's address type the amount etc.