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.