The problem is not with Android its with Google But apparently android is from Google even though its open source. Sure, linux phones are nice but they are nowhere near stable.
The problem is we do everything with mobile phones now even banking and security is a big lead here The Banking apps may need to make sure each and every linux phones are secure and may not have any loopholes Which is tedious Even installing a custom rom can hinder the default security sometimes. If any problems arise neither linux phones nor banks can give any warranty As of now android is more secure and stable and theres a lot of time ahead of linux phones to succeed