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.
Maybe that could be a reason for using a lot of wallets for people but i guess mostly people use more bitcoin wallets than just one or two because they don't feel safe keeping all the coins in one but they split them in parts in different wallets so that if something happens to one others should be safe.