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.