The actual problem of BTC is, in my opinion, weak psychological perception, while fiat money have adorable effect. Most people got used to fiat money just because they believe in its reliability (though the reliability of a y fiat currency is far not that strong), and one more thing for psychological acceptance is good infrastructure: for fiag money there are ATMs, branches, all the shops/banks/employers are in favor of working with fiat, and there is no such infrastructure for BTC: none of financial structures are ready to work directly with bitcoin. While the system remains the same, BTC will remain no more than a way to earn fiat.
I think it is not reliability or anything like that, it is not even "liking" fiat, I think it is just the wide adoption. For example if you live in any nation, the fiat of that nation would be accepted everywhere, doubt it would be possible to go anywhere and not be able to pay with that fiat, whereas we all know crypto is not like that right now and that's an issue. All in all I would expect that the people who end up not really using it would be the people who go towards crypto anyway.
So, when it comes to crypto vs fiat, the biggest difference and the reason why people prefer fiat is the fact that it is already accepted everywhere and can pay anything you want with it. As long as crypto goes up in adoption, we will get there.