I believe that is because of the very limited SM capabilities when compared with the Tau, apart from certain zany lists (13x THawk and a LC, 15x Landspeeders etc) which still have major weaknesses. Furthermore the Marine lists are pretty much set in stone while the Tau are still under development.
Going from 6x HH to 4x HH per cadre would permit a maximum of 11x AHMC formations (+ upgrades for BTS/SC etc) which is not a 'popcorn' list as such, but it is very powerfull with the only real weakness being against the air-heavy "drop" armies and teleporting lists which can get up-close-and-personal very quickly. It would also be very boring to play with or against (being the archetypal 'gun-line' that many despise).
Dedicated tank-heads can still use the current list to take 7x AHMC with 375 left over for relevant upgrades, and it would be interesting to see if this is OTT or not (though it would still be boring IMHO).
