Well I have seen stats comparing different areas and market shares. I admit I didn't save the data to my PC, but it shouldn't be that difficult to look up if you doubt me, basically, PC gaming is strongest in Europe while in the US the gaming consoles have a much bigger market share.
That in itself of course proves nothing, I'm sure none of us has a detailed sales potential analysis of a hypothetical PC football game lying around, but it's strong evidence that a PC american football game would have a much smaller market than a comparable soccer game, simply because you will sell FIFA in Europe, but not Madden. American Football, while it has a growing fanbase over her, is still very much a niche sport, there are still way more people into basketball and ice hockey over here rather than american football (and of course none of those come even remotely close to rivaling soccer), so that's a good reason why it's more profitable to make an NBA PC game rather than Madden.
Add to that the fact that after 4+ years, many of those people really interested in Madden who didn't have a gaming console back then, may own one by now (like me, I have to admit that Madden was one of 2 main reasons for me to buy a PS3) so those are even more people you would gain nothing from by making a PC version.