Even with decontamination procedures (that they haven't practiced in generations) there is still a chance their warriors will die from a bioweapon and clans abhor the thought of that. Even solhama, despised as they are, are supposed to die in battle, not the sick bed. Their mission is a race to Terra, with minimum distractions and a dead world is an unnecessary distraction, until Comstar explains to them that it is actually a very much alive world, of great strategic importance. This might give Weber time to prepare for the attack.