As someone pointed out. Liverpool and United have a core (as we do) of supporters who go to home matches.
They both also have a legion of daytrippers and Out of towners, as confirmed by various events - Liverpool the ash cloud crowd, and the United fake bomb scare crowd.
This is due to fairly recent history (the 60's onwards for United, the 70's onwards for Liverpool) - more mobility of population looking for work, more cars etc, and of course the temptation of following a team that is winning the shiney stuff. If we continue this current period of success, then City (as mentioned by many other threads) will continue to grow the young 'new generation' of supporters, just like United, Liverpool did earlier, and Chelsea have done more recently.
I'd also say (judging from observation, rather than hard fact) that the core Liverpool support are closer to the core City support in terms of the majority demographic of match going fans - 'working class', poorer, local, than the legions of the Cheshire set that pour into Old trafford.
This money helps supporters get to the ground and away day events - obviously.
So,again, as another pointed out, it's not realistic for many of the core City fans to spend on every season multiple trips to wembley (amazing to remember the 90's...), as it's a long bloody expensive trip from Manchester for them. Compare to the matchday migration North from London for United (and to a lesser extent Liverpool), it's a doddle and far cheaper to just hop on a tube to Wembley.
In time, a generation, possibly 2, then we'd end up with the same demographic mix as United and Liverpool - spread out, richer... but hopefully (please!) not so bloody arrogant and entitled.