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.
thats true, people use different wallets because of a wide variety of reasons and i think it is not just because of anonymity
Its of course for multiple reasons but I think the main reason is for security if you have it spread on multiple wallets its way more secure.
Anonymity is also playing a role but not a big one.