Because all the biggest clubs are interested in him. Apparently he's a main target for all of Barca, Real Madrid, Juventus, and Bayern, and he's more casually been linked to Spurs, City, and Liverpool as well
According to tier 1 journalist (Freek Jansen for Voetbal International, the most reputable Dutch football magazine), Ajax turned down a €65 million offer this summer.
He's only going to be worth more now, so how much are you going to be paying to make a January move worth it? €80m? Why wouldn't we be able to get that in the summer? Unless you're going (well) into €100 million I don't see why Ajax would even consider the deal.
Then there's De Ligt, who's got a very responsible head on him. Being a panic January buy as United try to save their Mourinho season isn't the kind of move that gives you any sort of long term security. Why would he go for the club that offers him that, regardless of him loving English football, United being the biggest club in it obviously (and he's talked about having red both Ferdinand and Vidic's autobiographies), when he can get a long term plan offered to him by any of those other big clubs, with probably the same money attached to it?
There's no way De Ligt risks the next couple years of his development for slightly more money now, when he's still only 19.
It's not that at all btw. I think you have great pull as a club, but it's always risky to commit yourself as a young player to a long term plan with a manager whose position isn't stable.