People use it because they are used to it and they don't want to switch until something bad happens to their funds , It's human nature.  Blockchain.info was probably the first online wallet that got first created , with time , people was recommending others to use it because It's easy to use and have a good interface , without forgetting that gambling websites was giving blockchain.info as an option to store their funds .And now It became a habit and no one gives a damn about their security anymore.
Unlike Coinbase and other services , Blockchain.info don't control our private keys and don't have our password stored in their servers but It's hackable , anyone with access to their servers could try to crack the wallet.dat and get some $$ in his hands with no effort as some passwords are weak.