quick and easy....
simple answer: Because most people naturally put a lot of snap and power into their forehand and need an overstable disc that they cant "turn over" or flip "sideways into a roller".
more complex answer:
On a more fundamental and philosophical approach... i've determined that instead of adapting one's throw to a FH disc, it's a lot better to find a disc that works for you; albeit understable, stable, overstable.. <whatever> ...as long as you can put it flat and drive it a good distance, it works.
my basic thought for a FH is simple... Walk up, dont think and "throw" (*Maybe 75%*)... not hard, not soft, but the "same every time"...
if the disc flips up (toward the face plate) and turns into a roller, you either have: too much power, not throwing flat (anhyzer - outer edge up), or too much of an understable disc. - Get heavier or more overstable.
if the disc flips down (toward the underside) and turns into a roller, you either have: not enough power, not throwing flat (hyzer - outer edge down), or too much of an overstable disc. - Get a lighter or understable.
The basic thought is that you should be able to throw a disc <table top> flat. Once you get that, discs can (and will) perform exactly like the backhand performance when throw correctly (ie. "s" curves, turnover, hyzer, etc.) The key is finding discs that you can throw flat - the "stability" is not necessarily the key element; finding the disc that works for you is. From there, you can adjust accordingly based on the performance you need.
Do note.... overhand (thumber, tomahawk) and FH throws are very rough on the human body. I try not to do overhand shots, and i use an elbow brace when i play as excessive FH throwing gives me "tennis elbow" like symptoms.
