The Inbox was a vanity project of Clay Bavor. He got the promotion he wanted and moved on. The official version, of course, was a different one and amounted to something like "more wood behind fewer arrows".

How does one develop these kind of cults of personality in large organizations that can allow for “vanity projects?”

Google seemingly axes everything half decent they make that then has a decent following and for seemingly no reason. I doubt the engineers who probably like the services they work on know why

I tried it and didn't like. At least it wasn't compelling enough for me to switch. I presume a lot of others weren't convinced either. I don't think there's a complex answer.

Meanwhile, some of its features made it to standard gmail.