The reason that it can’t offload more than once is that if it did so during combat movement it would be involved, though indirectly, in multiple combats. This would require complicated rules, as there would be cases where it could be done and cases where it could not be done. Transport movement is already complicated enough without adding more exceptions, so it was decided that a transport’s movement would end once it offloads, period, no exceptions. It was felt that any flexibility added by allowing multiple offloads wasn’t worth adding more complication to transport rules.