The problem is that people prefer convenience over safety and security and sadly, the decentralized exchanges are not as newbie-friendly and easy to use as the centralized ones and that's exactly what pushes people away.
And that's it
you already made the point that is needed. Centralized exchanges are very easy to make use of, even someone who is still a newbie will understand everything about it on the first day they are making n use of it and that's because centralized exchanges functions the same way as traditional brokerage/stock markets. So, they are easy to grab than the decentralized exchanges that are a lot hard to understand.
I don't even think this should be a problem to people, we have centralized exchanges and we also have the ones that are decentralized, so these shouldn't be a problem at all, cause you're free to make use of anyone you like. If an exchange decided to become centralized, then it's their choice and you're free to move your money to another platform that is not centralized. Binance, coinbase, Kraken, Bittrex, Bitfinex and other top exchanges are centralized and they are easy to use, especially Coinbase.