I have theory about this. Work was started on a new Elder Scrolls game a decade ago, but put on hold as those people were needed to get Starfield finished. Work resumed when Starfield shipped, but with so much time having passed it was largely a case of starting again from scratch.
The thing with a numerical title in a series is it automatically goes to whatever comes out next. So, there will eventually be an ES6. But there may well be several aborted potential ES6 games in between ES5 and whatever game eventually makes it to publication.