Just a guess, but my guesses would be
1) Too much horsepower for the 12x12 transmission
2) Too much horsepower at the wheels (maybe the tractor can't handle it)
3) Its the top of the line 3x20 series, so it has to have a hydro for image reasons.
But given the fact that if you get a mid-pto on the 3270, you have to order the air seat too, I'd guess its reason #3.