Maybe because others got much better results than you suggest to have gotten?
This is not really relevant to me; first, because the results other people got with different equipment don't have much relevance for my own experience and workflow. Second, because I am highly skeptical when someone reports results so far from what I am used to seeing.
The fact is, only the large format seems to hold its ground against 35mm dSLRs. Medium format isn't worse, it's just much harder to get the results. You need to have a Nikon 9000, and then you need to keep the film flat, and recently it's even difficult to get proper processing for E6. So, from what I saw, I'd have to invest in an Imacon or a Coolscan 9000 to really get the kind of sharpness and clarity I'm routinely getting from digital. And the thing is, I don't think the film is better. I am actually getting better landscape colors from digital, I prefer the end result. For portraits, I must admit I sometimes prefer slide film, but not by much, and I only find E100G to really be in the game for me.
As I said, when everything is just perfect, medium format film has a slight edge over my digital equipment, but it's only slight, which means invisible in print, but it is so much easier for me to get good results with digital, I find the additional investment of time and work just unacceptable. I have no problem working hard for the results if they are worth the trouble, but in the case of film, I don't feel they are. For instance, I recently ordered prints made from my 645 Reala negatives, from the only remaining E6 lab in Zagreb. I ordered A3 prints. The results were in every way inferior to the A3 prints I made with 5d. And I mean in every way - colors, detail, tonal gradations, general impression of the image, everything. It's not just a matter of some tiny fraction of resolution, but the tonality of the digital image looked vastly superior. OK, someone will say Reala is crap but I don't really think so, it's actually rather good for landscapes, as C41 films go. I could get better colors from E100G, E100VS, Provia or Velvia, but the dynamic range would be so thin it would be difficult, if not impossible, to shoehorn the scene inside it.
I did at one point feel that film is superior in colors and tonal gradations to digital, but that was when "digital" was an old generation small sensor digicam. With Olympus E-1 I stopped shooting film for all intents and purposes, because it was the same or very close in tonality and for smaller prints, up to A3, the difference in resolution that still favored 35mm film just wasn't significant. But with 5d I don't feel the lack of resolution even in B2 prints, which is the largest that I actually printed. The prints came out very detailed, clean and with perfect colors, and this is the point where I think I finally made my mind about film, even large format. The thing is, I don't feel I'm sacrificing quality by not shooting film. I'm consistently getting the colors I want, and I'm getting results printable in very high quality in sizes as big as I find practical.
So yes, I believe one could shoot a frame of 645 Velvia and expensively scan it on a drum scan and show that it outresolves a 35mm dSLR, but this workflow is not even practical for big studios, and certainly not for me. I have a 4990 Epson and could, if need arises, go out and scan something really important on a drum scanner, if the graphical studios who used them didn't switch to digital in the meantime, but the reality of what I am seeing is much greater difficulty of getting the results that are often not even there.