I think it has to do with his age, because now many big clubs are looking at younger players, and when they have a goalkeeper who is senior enough it is a goalkeeper who has been at the club for a long time. David d Gea is not that great, but his ability is still reliable. Manchester United did something hasty, so they did not extend his contract. The fact is that now they have a goalkeper who has no better performance than David d Gea.
Manchester United made a hugh mistake when they allow their first choice, David de Gea to exits Old Trafford this summer after failing to meet his demands simply because they had their sights on Inter Milan goalkeeper, André Onana who was at his peak but alot of things have changed since United signed him, he haven't performed to the expected assumptions of the fans. Meanwhile David de Gea is currently a free agent this season with no club. He's keen on making an impact move to an elite team because he's a top goalkeeper that has the presence of other top competitors, moving to an average team will only see him beckoned to mediocre level which he will totally refrained.