Why not? If something catastrophic happens, will it really make a difference?
Because if a fire starts, you'll have zero time to get out. In the rear, you may have a second or two.
Putting it at the back isn't always a good idea - Ford Pinto anyone?
There are many hatchbacks safer (ahem) than the Pinto. IIRC it had a particular design flaw in addition to the gas tank in the rear, though I've lost the details and wikipedia doesn't seem to be helping.