They use MF because they destroy anything 35mm can offer for what they do. Period.
Sync speeds, leaf shutters, massive sensor size, exceptional glass, and a sense of seriousness for client PR.
I find it amusing when some compared high MP 35mm to MF, its no comparison at all.
I don't think I really agree with all of that. I think that statement was entirely true five maybe six years ago. There is still a gap, for sure, but the gap is closing. The D800 has demonstrated that from a sensor standpoint, 35mm can approach the pixel counts of MFD. The D800 has also demonstrated that 35mm can far surpass the dynamic range of MFD. Medium Format glass is great, but so is the more expensive 35mm glass, particularly from Canon. The two are at least on par...and I would offer that Canon's latest Mark II superteles have higher resolving power with higher contrast than MF lenses (keep in mind, it is more difficult to correct lens defects and aberrations in lenses for larger formats than for smaller formats).
I can't disagree about leaf shutters, they definitely have some advantages, particularly sync speed. If you are a heavy flash user, which is particularly common in a studio setting, a leaf shutter can be a godsend. There is also no question that MFD cameras have higher pixel counts. Pixel count is frequently the most important factor of IQ...the more pixels on subject you get, the lower the relative noise, the higher the overall detail. In that respect, the need for lenses with similar resolving power to Canon's is somewhat unnecessary, MFD lenses resolve enough detail to support the pixel densities found in medium format sensors, and at the closer distances MFD is usually used for, such as studio photography, there is little contest at the current time (pixel counts currently trump lens resolving power).
That said, pixel counts in 35mm are increasing. It seems Canon is testing 40-50mp FF sensors in their next studio and landscape camera. In the next four to five years, we could see 60mp FF sensors, if not more. There are a few decided advantages to FF that MFD cannot touch: High ISO performance; Advanced high-speed AF systems; frame rate. With hyper-parallel readout technology, it will be possible to read out very high pixel count sensors at high frame rates. (Canon already demonstrated a 120mp sensor with a 9.5fps readout rate!) When you NEED those things, then the leaf shutters and massive megapixel counts of MFD don't solve your problems. There is no comparing an MFD to a FF DSLR...the DSLR wins hands down every time in the high ISO/high frame rate/AF tracking scenario.
So...I would say it isn't as easy to matter-of-factly state these days that MF is the vastly superior camera, no comparisons. There ARE comparisons, and in many comparisons, 35mm comes out on top. That clearly indicates that MF, while it still certainly enjoys a for-the-moment-untouchable prestige in the studio photography arena, and in many cases the landscape arena, its powerful edge is dulling. In the landscape arena, where MF once reigned supreme, the D800 has REALLY closed the gap. It still lacks in terms of pixel count...one could photograph landscapes at 80mp if they wanted, or even 200mp with hassy's multi-shot mode. The vaunted D800 still can't quite touch that. The dynamic range of the D800 seriously brings into question the benefit of MF for the average landscape photographer, however. The studio prestige you acquire with your customers when you haul out the MFD doesn't exist for landscape photography...people care about the scene, not the equipment used.
To my knowledge, all medium format sensors still have a lot of read noise...similar to Canon's at low ISO. It will be interesting to see if medium format cameras move up from 11-12 stops to 14, or even 16 in the few cases where medium format offers 16-bit conversion (I believe Leaf has a couple 16-bit backs), with new advancements in sensor technology. Their key edge was pixel counts...with greater sensor area, they can pack more in, at similar pixel densities as smaller formats. There doesn't seem to have been much innovation on other fronts for MF sensor tech. If they do solve read noise problems and move up to ~15-16 stops of DR, MFD might survive the onslaught of DSLR innovation for another generation or two, assuming the DSLR market doesn't also move to 16-bit as well.
MFD is not the unassailable ivory tower it once was. There ARE comparisons, and the gaps ARE closing. Competition for the studio space will heat up in the coming years, and the MFD market won't be able to solely rely on "prestige" forever.