Seriously, we’re all just speculating almost blindly about this one. Meaningless.
On a less serious note, here is my guess:
Gomes was one of England’s two biggest talents (with Sanco) at 16, mostly playing 18-year olds. Mou gave him a start because he deserved it, and Mou wanted to look like Fergie, but wasn’t really thinking about the kid’s development path. After that, Gomes hardly got a minute under Mou, not surprising given the kind of footballers and play he wants. He probably was disheartened in this. During the same period, Gomes had a few injuries, and developped less physically than hoped, while players like Foden Brewster and Hudson-Odoi in the England set up and in progress towards grown up football.
Gomes was always a captain, so his attitude towards coaches, training and trammates must be very good. As I remember, already in the U-23 team his one weakness was that he would often let the game pass him by, evenif he shone there as well. This tendency of being quiet we have seen on his quite a few occasions for the first team. There was also the question of what kind of player he would become, playing against kids, he would be a top scorer, at higher age level he couldn’t as easily manage the goals, and has even been played in midfield. Maybe he and his advisers see differently what kind of player he should become Butt, McKenna and Solskjær.
Gomes got game time early on under Solskjær, together with Chong and Greenwood. He has seen them and several others move past him in the queueline, while Lingard, Pereira and Mata have blockednhis path with less than stellar performances. At the same time, Gomes hasn’t actually been able to displace them with performances.
As I understand, his father Gil, the former footballer, has been his main agent, and I’m guessing his dad has watched from the sidelines and have been disappointed with the relative progress Gomes has made since his debut at 16, and three years later, a plan to ease him into first team football over another two or three years has made him think his son should try somewhere else.