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.
It seems like the incentives for employees at Google are the problem here. People are rewarded for successfully completing projects, not for maintaining existing ones, so rational employees will maximize their compensation by moving on from a project after it launches and trying to launch a new one.
There seems to be nobody at the top enforcing a consistent product vision.
This is how Android ended up having several text messaging apps at the same time a few years ago. Apple certainly would not have done that, and I doubt even Microsoft would have allowed that state of affairs to exist.
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.