You would think that, but the fact is that anything that can stun an av12 vehicle requirees s6.
s6 would insta kill one of the HWTs and force a morale check.
The amount of firepower that is needed to average 1 destroyed Chimera in a single turn is more than enough to wipe out an entire HWS. It takes 12 lascannon shots per turn to probably blow up a Chimera - 12 to shoot, 6 to hit, 3 to penetrate av12, and 3 to roll a destroyed result - and your HWS would still be alive. Think what shooting 12 lascannons at a naked HWS would do to it.
For 130 points, you get more firepower than a Leman Russ Exterminator, can shoot at 2 different targets and can score, the only disadvantage being a lower AV. I can see why they didn't do it.