All it took was City, PSG, and Chelsea flashing cash despite not being top clubs, and the likes of Agüero, Zlatan, Thiago Silva, Ballack, etc. jumped ship.
United's issue seems to be trying to purchase players who are happy at elite clubs and/or the clubs having no need or desire to sell. If the club decides they want players like Alex Teixeira, Felipe Anderson, Sadio Mane, Romelu Lukaku, etc., they're in play for sure. The issue is, they will have to compete with the other big clubs and pay over the top fees and wages to get them, which they seem more willing to do these days. Chelsea, City, Real, and PSG have completely changed the market, and United seems to be adjusting.
Every club will have its appeal to certain players, though. English weather likely doesn't appeal to players used to heat year round. A club like Madrid has had the Galactico reputation for the past 15 years, which makes it an honor for players to be considered by them. This was accomplished by being an established club who spent over the odds on players. This can be accomplished in England if it's what a club is looking to pay for. Paris now has the appeal that it is an easy league with a team competing for European honors.
United, though, is one of the richest clubs in the world competing in the richest league in the world that will be, if not already is one of the deepest, most competitive leagues in the world. That's quite an appeal to competitive players who don't have an issue with the area. The big problem there is that Britain, Ireland, Iceland, and other countries with smaller leagues that would be attracted to life in England aren't producing elite talent at the rate of Spain, Germany, Brazil, Argentina, etc.