Without wanting to turn this into yet another "Liverpool" discussion...
It is a true statement that the demand for rail travel to/from a town/city is often in no way proportional to its population size. Demographics play a part indeed (don't read that as claiming MK is middle class whilst Liverpool is not.... MK has its rundown parts whilst greater Liverpool has some very affluent areas, for example, but probably much more spread away from the city in general), as to where the city naturally looks to for business links.
So MK is linked to London, Birmingham and Manchester for it's 'graduate' type employment (handily being on line between these points), whereas Liverpool traditionally looks towards Manchester for its Business/Employment/trade* links, probably moreso than London.
*Probably tracing its roots to the Manchester Ship Canal and Liverpool & Manchester. But London<>Manchester linksprobably stem from the Cotton trade.
Time was of course when Liverpool's economic links extended country- and worldwide, albeit Manchester was always an important trading partner. Deindustrialisation of most of Britain, and over-centralisation in London, has sadly wiped out many of those links, but that's true of most other British cities outside London.
Note that I did not deny that MK is a major destination and significant generator of rail travel (though I maintain that it is, at present, a poor rail hub because of the lack of connections, which is how this discussion began). However, I don't think I'm unfairly denigrating MK to point out that there is no basis in fact for some of the claims made about it. And I wasn't the first person to bring Liverpool into the discussion: that was Bletchleyite, with his prediction about MK somehow growing to outstrip a city it is currently somewhere between a third and a seventh the size of (depending on which definition you use), and with his incorrect assertion that Liverpool's population is in decline.