IMO each wallet has advantages of it's own, And because of these advantages people use different wallet according to their needs, For example:

-  Armory is secure, People with cold-wallets or big amount of BTC use it
-  Electrum is fast, doesn't need to download the whole blockchain and PC based mostly
-  Blockchain.info is fast and incredibly easy to use, suitable for beginners and small amounts of BTC

And the like, each wallet has an advantage of it's own.