Why 24-70 (or 70-200 for that matter)?

Slightly odd question, but one I was just wondering about.

Why is the 24-70mm focal length range (and the 70-200mm to fit with that) generally regarded as the 'standard' focal length range for a decent zoom?

Obviously Canon and Nikon premium zooms are in these focal ranges, and the new Panasonic m4/3 zooms are as well - just wondered if there was any particular reason, or if it was just someone did it first and the rest followed and then it became standard!

Cheers!