It's been like this for as long as I can remember tbh. Liverpool aren't too far behind in this sense actually, but United are and have been throughout my lifetime the biggest, most talked about, most referenced team. Opposition fans remember their victories against United with extra joy, they moan about the defeats with extra bitterness.
The clearest example I can recall, was one year when Chelsea won the league. 2010 I think. They won the league after their match, and Sky spent about 5 minutes talking about Chelsea's title triumph. They then spent about an hour talking about United, and another hour talking about Liverpool's top 4 hopes.
Also the commentary. When United were winning most weeks, every half chance that an opposition created was bigged up as a glorious chance, a result of the oppositions brave efforts. Every chance United missed was hailed as a result of the oppositions staunch defence. If United ever scored a winner/goal to take the lead later than about 65 minutes into the game, the commentary would always suggest that United had stolen victory, and the opposition were so unlucky as they had defended so well, despite United having spent all game battering them and peppering the goal. The tired line "hallmark of champions, grinding out results despite not playing well" - when we had had about 20 shots to the oppositions 2, and the opposition had spent he majority of the game camped inside their own half.
I think we're receiving somewhat similiar media coverage to Liverpool in the late 90s/early 00s. It was always, can Liverpool recover and launch a title bid, what has gone wrong at Liverpool etc etc. The platforms and medium has changed, but the focus is the same. United first, Liverpool second, Arsenal a very distant third. Nobody cares about City, nobody cares about Chelsea, nobody cares about Spurs. Everyone else is an irrelevant plucky underdog.