It's also well documented in a University of Liverpool study, that the building of the Manchester Ship canal was the start of the true rivalry between the cities of Manchester and Liverpool. People in Liverpool lost their jobs as ships by-passed them and made their way to Salford docks. Football only exacerbated and exaggerated this.
Okay, let's deal in facts... Yes Manchester & Liverpool are rival cities. There are many reasons for this with each city's stance during the American civil war playing a part, as well as the rapid development of Manchester and its challenge, in terms of northwest dominance to Liverpool etc.
This rivalry was not obvious in footballing terms until the modern era. Sure Manchester clubs and Liverpool clubs were rivals but not at the level of a true derby, whether that be in either city.
More facts... If the Utd-LFC game has always been perceived as a derby, then why isn't this obvious in contemporary evidence, either in match reports or in actual match attendance? Take a look at the early 1950s when MUFC enjoyed a golden age in terms of success, with crowds growing and attention on the club reaching a new high. This was also a period when Liverpool reached a FAC final. What were the crowds? What? A little over 20k at old Trafford (which could hold about 65-70k) for such an important ‘derby’ game in 1952-53 when Utd were reigning champions as well? Surely not? Was this a blip?
What about 1953-54? 26k at OT! MUFC's 3rd lowest league crowd of the season – but this is ‘the’ game!
What about 1951? What? LFC at MUFC attracted 34k, Utd's 8th lowest league crowd of the season. Surely not? What about Everton, after all they are also a Merseyside club so the rivalry between Utd and Everton must have generated an incredibly high crowd – it was c.29k! How can that be when Manchester-Liverpool are such great rivals?
I could go on, but this is a perfect example of how modern day rivalries may not have been what the media and others claim they are.
As Prestwich Blue stated, why weren't City- EFC games perceived in the way you claim MUFC-LFC were? City and Everton were the first successful sides from their cities and did have several grudge matches in early 1900s, however when City won FAC in 1904 Everton proudly displayed the trophy at Goodison for all to see when the 2 sides met. Everton stressed their support for Lancashire's success!
As for the comment about managers… I’ve never found anything from Busby talking about the great ‘derby’ with LFC. He talked of facing his old club etc. but never spoke about the game in terms if a derby, unlike his comments on the Manchester game of course. Busby had understood Manc football from his arrival, so if it has always been seen as a true derby then why didn't utd’s great manager and former LFC player mention it? Surely he'd have talked of the great ‘derbies’ he played in?
LFC-MUFC is a special fixture, just like City-CFC is becoming, but so are MUFC-lufc and MUFC-Arsenal. Those sort of games get hyped up and have an edge, but they're not as significant throughout history as the Manc derby. Utd’s footballing rivals in 50s were Wolves, City's in 30s were Arsenal, but that doesn't mean those games remain more significant than the Manc derby