Because a red headed, tow cammed torque monster will smash most manuals available for ready & easy fitment into a HQ. Aussie 4spd's and Celica/Supra boxes aren't really up to the task. They'll last if babied, but why would you bother with a 350ci stroker if you couldn't give the throttle a blip from time to time? HSV removed T5's and Getrags as they weren't strong enough for the 5.7/350ci Holden engine. Leaving a Muncie or Super T10 or one of those 5spd Toploader things as the only really affordable manual options that fit without serious tunnel mods.