"Manchester United Versus Liverpool is the biggest game in the Premier League. In fact I'd say it's the biggest Derby in Europe"
Some no nowt on radio.
Thing is, it is a very big game. In the same way that people who aren't actually all that into music will pay attention to Coldplay and BTS, people who aren't particularly into football will often "support" United or Liverpool.
Pumpkin farmers will big-up Halloween because it's their opportunity to reach an otherwise inert audience (and hopefully sell more product at other times of the year) and similarly, the general "Sports Media" (for want of a more accurate term) will big-up El Plastico to try and ramp up the price of advertising.
The really sad thing is that with the demise/deliberate demotion of the FA Cup (by the Premier League), this fixture has been inflated, way above its true sporting value. More often than not, it's a dreadful spectacle that attracts nobody to watch another game and unlike say, Leicester catching Chelsea last year, it is completely devoid of any surprise or romance.
The Premier League and the Sports Media would probably offer the Thatcherite /Reaganism argument that a rising tide lifts all boats and therefore everybody benefits from the promotion of this fixture.
If we accept that though, we must also accept that a guiding (if largely unspoken) principle of Thatcherism is "to let the devil take the hindmost". So, we could argue too that far from "lifting all boats", clubs going down the toilet (City were nearly one of them, remember*) is one cost of this fetishism.
*and have since been vilified for daring to undermine fixture's standing - but that'd be tilling old ground in terms of this thread.