They have to make a certain power factor (bulley weight x velocity = PF) and 9 would be too small and 45 is fine, but a little bigger than it needs to be and magazine capacity starts being an issue.
The IPSC motto is DVC (diligentia, vis, celiratis) Accuracy, Power and Speed.
IPSC shooters will also employ 38 Super, 9x21, and even 9mmMajor (if they're using Glocks) to find the best combo of bullet weight/speed and recoil.
Regular old 9mm is making a big come back with the new-ish "Production" division.
You'll see a lot more 45 next year thanks to them copying IPDA CDP (1911's) division and calling it the "Provisional Single Stack". Currently people are using 1911's with those 10 round magazines in the Limited-10 division - and I for one am not a big fan of the 10 rounder.
If you liked what you saw - you should swing by and check out a local IPSC/IDPA match. You'll see how us petty humans shoot (a little slower than the dudes on OLN). Be careful you don't get sucked in like I did.