The new (good) ones have to be better than those they are trying to replace. In every sense of the world. If it's a combination of good and bad, most people will just continue using the sites they are comfortable with. Unfortunately, it mostly boils down to how much money/crypto will eventually end up in someone's pocket or wallet. You are not going to get to most people telling them how decentralized exchanges are better and explaining the reasons why that is, when all they see is that the centralized one gives them $50.000/coin, while the best offer on the decentralized one is only $49.200.   

Then there is the issue with marketing. Have you ever seen an ad or commercial for Bisq? Me neither. But if you watch sports, you can see centralized exchanges in the biggest sports competitions nowadays. Money buys new customers.