Simply, Centralized exchanges are easier to use for beginners than decentralized ones and have more advanced tools. In addition to that, they offer tech support unlike DEX  and by using their platforms you are allowed to buy crypto with FIAT. In fact, crypto users especially newbies are more looking for simplicity through easy operation and regulation. Regrettably, centralized exchanges have a plenty of cons  such as risk of getting hacked, no anonymity, your funds are completely controlled by them.
As far I am concerned DEX’s have not been known well yet due to they are newer than CEX’s. Possibly more Bitcoiners or generally crypto users will move to DEX once they get to know the advantages of them.