Because they are THE glory-hunting creeps club. The bloated monster that sucks up the feeble-minded and impressionable. No other British team has tens of millions of "fans" in China, or someone in nearly every tiny English hamlet who supports them "because they're good and they win things". Listen to any football phone-in and you'll hear accents from every corner of GB talking about "us" when they've no more connection with Greater Manchester than most of you have with County Durham.
Liverpool in particular, and Chelsea, are bad, but no-one comes close to the loathsome manu. Never have I wanted you to win a match more (other than when there's money on it) than in 1983, but The Devil looks after his own unfortunately.
I genuinely don't think there's that much of a difference. Asia is absolutely rammed full of Liverpool fans, and the number of Chelsea (and I'm sure Man City) is growing rapidly. I'm not saying that there isn't a huge presence of Man Utd fans across the world, but to single them out as 'THE glory-hunting creeps club' and a 'bloated monster' is taking your hatred for them a bit far.
Whatever you say about Man Utd, at least they have made their money through hard work, perseverance and ultimately, success. Less can be said about the like of Chelsea and Man City.