These are not only the reasons fast, secure but there are some other reasons too that are low transaction fees, increasing value while fiat value do not change like Bitcoin value changes.
Increase value in fiat and bitcoin is different because of their usage globally. Fiat have a stable price because the usage is balanced and well performed unlike bitcoin which is more presently active in trading and investment and not as how usual currency works is.
Actually Im not with the point why people prefer bitcoin over fiat because of transaction fees. Majority does want bitcoin because of it's fiat value that is increasing over time.
Several people does not want to be bothered with identity stuff, or evading bank system every time they want to transfer money to relatives or to other wallet address or account. Aside from that it is safer to use bitcoin in transactions than using visa card or credit card in terms of payment information security.
I am sure that people not involved in Bitcoin find fiat payment services far more interesting and safe to complete payments than is the case with Bitcoin.
Mainly because of the fact that their purchases get insured against theft, damage, or whatever other inconvenience that they may face.
Bitcoin can't offer that level of security to people that are used to these security features. That's why they will prefer to keep using fiat instead of something else.