Statista chart here -> https://imgur.com/GwRTnEx

As for me, I see 2 biggest problems which are not mentioned here :
1. incredibly long and complicated verification process when you want to deposit fiat money: you should provide tons of your personal documents
2. problem on the banks/ processing services side - you should wait days before exchange will receive your money

from my point of view it should works like any eshop or service, you paid money and immediately receive your btc to wallet.