Because his profile was raised by PSG's interest, which seems to have been largely based on their FFP restrictions, and this has been followed by Everton doing the old Moyes trick of hitting form when there's nothing to play for. That's all conjecture, obviously, but it's my take.
United are a club that doesn't know what it's doing in the transfer market, and Everton should make the most of any opportunity to take them to the cleaners before they bring in a competent director of football. They're about to lose Herrera and Pogba, and have had their fingers burned with Fred, so they could well see Gana as a safe bet in the short-term.
He isn't a terrible player, but nor is he anything special, which is why I don't think that it would difficult to find a younger, cheaper replacement.