For the same reason that the world largest tire manufacturers such as Bridgestone/Firestone and Michelin say you should - safety.
In the old days the "common sense logic" dictated that, especially on a FWD vehicle you put the best tires on the front axle "for traction" and in particular for wear since the front tires wear faster than the rears.
What was meant wasn't really traction though but rather "acceleration", which is very different. Actual testing has shown that if the tires on the front of the car have more traction than those on the rear - such as if you put 2 new tires on and mount them at the front - the car is many times more likely to spin out of control on a wet, snowy or otherwise slippery road. For "vehicle control" you want the best traction on the rear axle. If the front tires start to slide the driver usually gets plenty of warning and the vehicle can be brought back under control relatively easily.
When the rear tires lose traction a snap-spin without warning is usually the result and chances of regaining or maintaining control of the vehicle are somewhere between slim and none.
The lawyers have even figured this out. Michelin was just sued for $34,000,000.00 last year because one of its stores put 2 new tires on the front axle and the driver lost control in the rain.
This is the point in the discussion where most people get bored, roll their eyes and say " but I've always been told to put them on the front so that is where I want them."
So here's your evidence of what I'm talking about. Scroll to the bottom of the page and click the link. It will only take 2.5 minutes of your life.
http://www.michelinman.com/tire-care/ti ... questions/