Because bikes tend to ride better that way, for a number of reasons.
Here's my attempt at explaining some of those reasons: most of your weight is on the rear suspension, and you have legs to help absorb much of the impact, so you don't need as much back there. A lot of suspension back there can also sap some of your pedal power, although that has become less of an issue with most modern bikes. Up front, if you're riding really rough stuff, a little more travel gets you a bit more forgiveness in the front end and a bit more capability. You lose less power with increased front suspension. Your front suspension has to work a little harder in big rough stuff because there's less weight on it, your arms aren't quite as good at absorbing bumps as your legs, and stuffing your wheel could mean OTB.
Bottom line - slightly less travel rear compared to front makes the bike feel a bit more balanced.