It's the lower frequency, longer distance for the same power. So a 500 Mhz vs. 2400 Mhz makes a big difference.
Oddly, this doesn't seem to hold true for 2.4GHz in other use areas. For instance, radio control hobbyists enjoy the same or greater range, with less interference, using 2.4GHz digital systems as opposed to the 72MHz FM and digital systems that are being phased out -- and the 2.4GHz gear operates at lower power.
I've been having to do a fair amount of research on 2.4GHz equipment lately for other reasons, and from what I can see, if there's a problem going on with interference between wireless routers and wireless mics, something is not playing by the rules with regards to the 2.4GHz ISM space.
Oddly enough the laws of physics still hold true even though some would think otherwise. 72 Mhz has more distance capability than 2.4 Ghz.
Anecdotal experience proves otherwise. Yeah, I know. Theoretically, it's a physics thing.
But, as Yogi Berra said, "In theory there is no difference between theory and practice. In practice there is."
And as an engineer working in television by day, and musician and working sound guy by night (and weekend), I can certainly attest to that. 
Cheers!
Well Gerenm63 there seems to be quite a variance in the RC crowd regarding the 72 mhz vs 2.4 gig rigs most still will use the 72 for longer distances. Most reasons are for other factors. Remember that the 2.4 gig received quite a bit more engineering talent than any 72 ever did. Here are some opinions pro and cons.
http://www.rcgroups.com/forums/showthread.php?t=1546959
And some realty crazy guys. Check out the flying microwave oven.
I've been fooling around with R/C airplanes and cars off-and-on since the '70s, and now use 2.4GHz stuff for control of model trains (one of my other expensive hobbies). I did some "stress testing" when I moved to R/C control, and had running about a dozen 2.4GHz R/C transmitters, the two wireless routers that live in my home (and I could also "see" the neighbor's router and the municipal router), the router for the DL (attached to the DL), two iPads, two iPhones, two laptops, and one iMac all on and working with no problems.
There are some old hold-outs out there in R/C land (my dad was one of them until just recently). And, yeah, there are some real nuts flying almost anything they can figure how to get off the ground. I've seen flying lawnmowers, pizza boxes, cars ... you name it. Maybe it depends on region, but I wouldn't say most are still using 72MHz, at least not around here, unless they're real curmudgeons or really strapped for cash.
