Probably because there simply isn't money to be made in being an e-mail startup.
Or, more correctly, there isn't enough money to be made to satisfy VC investors. Mailbox and the like could have lived quite comfortably as paid-for apps with comfortable but small levels of profit, but that isn't what Silicon Valley rewards.
EDIT: also, IIRC Apple's restrictions on background processing meant that they needed to have every user's account set up on their servers to process push notifications. Did that ever change? If not that's a non trivial expense/complication to scaling, especially when you're up against an OS-integrated app that doesn't have those restrictions.
Or, more correctly, there isn't enough money to be made to satisfy VC investors. Mailbox and the like could have lived quite comfortably as paid-for apps with comfortable but small levels of profit, but that isn't what Silicon Valley rewards.
EDIT: also, IIRC Apple's restrictions on background processing meant that they needed to have every user's account set up on their servers to process push notifications. Did that ever change? If not that's a non trivial expense/complication to scaling, especially when you're up against an OS-integrated app that doesn't have those restrictions.