What could be the reason? it's because their poor server can't hold high traffics?
At least 80% of the Bitcoin/crypto users aren't that tech savvy. I also think that at least 80% of all crypto users prefer convenience over safety. Maybe dealing with a centralized entity dives them a false sense of security. It's like having a bank account in a big and trustworthy bank. You think that all your problems will be solved and you trust the bank, simply because the bank is big and it has millions of users. The same thing applies to big crypto companies like Coinbase. In this case it was a technical glitch, but I really expect some big centralized crypto exchange to get hacked again and many crypto supporters waking up with $0 in their accounts.
Speaking of convenience, both of them are not really different.
When you use centralized exchange, you don't have to login because you've download their apps and login into your account, you will save your email and password in case you're forget.
When you use non custodial wallet, you also don't need to login anymore because you've import your seed phrase, and you've save your seed phrase.
The reason why people choose centralized exchange over non custodial wallet because they can stake their coins to earn interest, they're choose profit over security.