Let's not forget that you can't put the entire ski up on edge to the same edge angle; the ski twists. The farther away from you the more the ski is twisted from the edge angle at the foot. The flare works with torsional rigidity to provide the effective radius at any point, providing the snow is hard enough and the ski is flexible enough to be bent into the shape dictated by tipping angle on hard snow. On soft snow it's pretty much flare and longitudinal flexibility working in concert.
Also if the tail were providing an effective radius smaller than the tips, it would cause the tips to dig in, but if the tail were providing a larger radius it would just ride a little higher in the grove while releasing some pressure from the tips, so smaller tails provide less friction.
Also it is easier for most people to control what's in front of them than what's behind them, so they put more of the working surface of the ski in front of the boot.