It's just that a lot of people find it unpalatable that a club is lifted from obscurity by a billionaire's largesse.
However, it's not like it can be done so easily everywhere, there is a challenge for the investor and the traditionaly dominating clubs can take measure's to discourage such upstarts or potential usurpers of the throne.
By nature regardless of what sector investors would invest in they typically would make an assesment as to where the largest "hole in the market" can be found as to yield them an optimal return on investment. Investors being investors most of them still seem to be in it for the profit afterall and making a healthy bussiness. To a large degree their core bussiness is to "reek a good opportunity when it presents itself".
It must be that the EPL presented more of a hole in the market for it to be so inviting for such investors. Perhaps it's understandable that they would be less inclined to try to crack for ex. the dominance of BM in Germany, the revenue's in Germany are lower and the required capital to be able to challenge BM might be larger than what you need in the EPL as well as that the risk might be considered higher for an optimal return as BM is a rather consistent team taking in prices year by year.
If it proves however that one of the best league's in regards to return also has a sort of leadership vacuum at the top with post SAF era then it is perhaps understandable that for any investor in this sector the EPL might prove to be the most lucrative spot for investment.
Well i can't fault the investors of city doing that, because they have exactly managed to do it all, make it a healthy club in value and return and attempting to take a dominant spot similar to that like BM in Germany or PSG in France. The ease by which even seems to dispense of the myth that the EPL would be so competitive to begin with.
And i think you can agree that money is just a part of the equasion to succes. You need more than that, good tactics, managers, trainers, morale, organisation etc. So it did require of the new city management to manage things well in all these aspects.
Manchester united was never forced to accept it all so easily and bend over to this though. Arguably they have just as much if not more money and/or attraction to be able to have dominating players/managers and overal organisation. However i think it's easy to argue that had united managed better to have much better overal performances and results in the last 5 years that it would have been much harder for any upstart to simply surpass it so easily.
It does surprise me that the oganisations of traditional teams which you'd think to be rather experienced can be so easily upset by the injection of a volume of money fairly similar to that of the top teams.
I guess regardless of other discussed matters that i'm rather impressed by the city investors abbilety to step into this market and seize it by the horns regardless of experience or history.
Perhaps there is an argument to be made about effecient management, also taking into account that the ROI that the investors achieved with city.
I guess you could say then that the city investor considered that United didn't spend its income effeciently enough to guarantee itself a dominant spot or resist the takeover by city.