That's a lot to do with other clubs having similar money to spend. That's the real reason they're not a force anymore.
Back when they were a force, they could outspend every other team. They spent about £30m on Ferdinand, what other Premier League could spend £30m on a defender near the turn of the millennium?
Andy Cole was the best goalscorer in the league at one point, playing for their nearest competitors...but United had the dough to buy him half way through a season. That would be pretty much unheard of today but they had more money than the rest to make it happen.
United still spend crazy money, but now they're not the only ones who can. That's the big difference.