I have a theory a factor in ETH wanting Mount is due to his ability to provide cover and flexibility with his ability to play a number of positions (RW, LW, CAM, CM). This wouldn't have been as pressing a need in the squad if Sancho, a player who in theory could provide options on both flanks and at 10, but ETH saw enough last year to know he wasn't a player he could count on.
Bringing in Amrabat, as well as ETH's apparent plan to use Mainoo in the first team this season, should allow us to use Mount more in attacking positions, specifically on the right, where we are short on numbers and quality atm. It's worth noting that in preseason, before being injured, Mainoo was selected in what was obviously ETH's strongest XI, with Mount at RW against Real. It's just 1 match in preseason, but at the very least it's a tactical setup he was at least considering.
Perhaps I'm thinking about this way too hard, but to me it seems Mount and Sancho offer some similar qualities and positional flexibility, but one plays and trains with the actual intensity, commitment, and focus that ETH demands. It would not surprise me if Sancho's lack of dependability which ETH witnessed first hand last year played into the thinking that led to United chasing and ultimately landing Mount in the summer window.