Volume is the most important reason and plays as main barrier of decentralized exchanges. Investors and traders don't want to join exchanges (centralized or decentralized) that have low daily volume...
I agree with you but I think that the main reason is that such exchanges don't offer a possibility to buy crypto using FIAT, credit cards and so on. I thought it will change when multiple stablecoins popped up lately but this is not the case. To be honest I don't know any decentralized exchange which offers trading pairs with stablecoins.
...A good example of such a transformation from decentralized to centralized is IDEX...
I wasn't aware of that. This is just insane, I wonder how they explained such a decision?