In short - because it stagnates. After few years of development there is even no official UI wallet for any platform. And the only official wallet at the moment (console zcashd) is buggy. After few years of polishing it this console toy is buggy. Just imagine!
Zcash is stated to be able of providing user with super privacy, but HOW? How should average user use this privacy? There are no handy wallets which allow it.
20% of block rewards is a huge sum. So why not to hire two-three more excellent programmers to develop user friendly wallets???
This are my reasons. For more I advise you to look into this thread.