Hi (ex-googler here, although i wasn't on this project). Product usage isn't a one-way street. It requires coordination from marketing, customer support, and ongoing product development. Inbox was yet another product that Google spent years on internally, launched with a bang, and then basically forgot about while the PMs got their promo and moved on to new projects, and so on.

To make a bad analogy (sorry) in response to your "lots of data and feedback", what Google repeatedly does is the equivalent of walking out into a crowd and yelling "HEY FREE HOT DOGS AT 1600 AMPHITHEATER PARKWAY" only one single time, and then concluding a week later that, even though initially the people who heard about the free hot dogs came by last week, since nobody is coming by for hot dogs anymore that people don't want free hot dogs. Remember Duo? Allo? Even Hangouts Chat from earlier this year is probably unknown to most people because google's stopped yelling in crowds about it.