Even though as consumers we would like to continue to have a medium-to-large lineup of camera bodies, in a seriously shrinking market Canon has to decide where to spend a shrinking pool of money to develop and manufacture new bodies. Hence I believe one of the reasons that they decided to try and bridge the gap with the EOS 90D; the gap between their APS-C XXD and XD series.
When a large group of consumers (probably a fair number of Pros, Semi-pros, and serious wildlife 'hobbyists') dearly wanted a 7D Mk III and others were wanting to move up in capabilities from the Rebel line but didn't want to have to replace all their EF-S glass then a replacement for the 80D would be the logical choice.
Canon is in a tough spot (all the camera manufacturers are) in trying to decide what lines will continue and which will go the way of the Dodo bird. I can't see their end game and don't know how much their R&D (for Camera and Lens development) budget has shrunk and will continue to shrink. But it seems to me that they perhaps realized a little late that they needed to get in the FF mirrorless game. And so devoted some hefty resources to that development at the expense of a 7D Mk III. I don't have any way of knowing, but I think within Canon they had to make the hard decision to either move forward with the XXD line or the 7D line. They looked at the sales numbers combined with market research and realized if they could only move forward with one of those models at this time that it should be the 90D. So they upgraded the frame-rate, the autofocusing, and a few other areas - more than they would have than if they were just replacing the 80D/77D.
I am waiting for them to make a quite refined, super-high-frame-rate APS-C mirrorless, with IBIS, a rugged build, and killer tracking focus. And a host of other features that one would expect in a wildlife/sports/action camera. Basically a mirrorless 7D Mk III - an EOS R7? Or 7R? ...? But we'll have to wait as Canon determines the market receptivity to the offerings they are already slated to launch.