The Bicycle Paper did a little study on injuries in cyclocross.
It has some interesting and some obvious results. Conditions play a role, but partially wet was more dangerous than totally wet a muddy. Not sure if their numbers are statically significant - my stats class memory suggests it's not, but still... Also Cat 1/2 actually had a higher rate of crashing than the 4s.
A few excerpts:
"Not surprisingly, the weather and course conditions that resulted in the highest number of injuries (16) came during overcast and partially wet conditions. Wet and muddy as well as partly cloudy and dry conditions were a close second, totaling 14 injuries each.
Category 4 riders sustained the most injuries (12 or 1.9% chance of getting injured) followed by Category 3 racers (9 injuries or 2.0%). These categories had higher participation numbers than Category 1/2, who had a 3.0% chance of injury."
Interesting study. I would think that injuries in 'cross would be incredibly variable. I also think that the higher speeds in dry conditions and the likelihood of crashes in wet conditions probably cancel each other out to some degree, though crashes in soft and wet mud are probably less severe.
As for ankle injuries- many of the run sections I've experienced were either flat, straight, or really soft or muddy. I can't recall too many times I've run where ankle injuries would be likely other than Charm City's tree root U-turn (and even that is such soft dirt that you just kinda slide around) or Granogue's wet and rocky creek-bed.
That's a really interesting study. One thing that's strange is how pervasive broken collarbones are in all forms of cycling. It makes sense that Cat 4 has the most injuries, both because of inexperience and packed fields.
My take on the lack of ankle/foot injuries is that we're not runners really, at least for me, when I have to jump off the bike I take it a little easier. I don't feel as confident off the bike so I don't push it and take the kind of chances I would when clipped in.