Opportunity cost seems like the biggest reason. Why devote resources to a project that is merely profitable when they could be working in something that is hugely profitable instead?
Reasonable, but then why launch Inbox in the first place? The best email client possible is not going to displace Gmail on a billions of users scale.
It makes a lot more sense from a corporate gamesmanship perspective. I think the incentives of the typical Google employee (widely believed to heavily value launching new stuff) are not in sync with the company or its users.
