They're used A LOT, especially by newbies so they're lucrative for an attacker to hack.
They're also closed source which means the wallet is developed by one central team and only they have access to the code. Wallets like electrum and bitcoin core do well because anyone can see their code, and, if they find a bug can report it and most can even trace where the error might be in the code allowing a developer of the project just to change it as it's reported.