P.S.
As for the reason why, it's because a freighter doesn't need to carry that much fuel
However, saying this, Boeing does offer the option of an AUX tank in the forward cargo on the ERF (and as far as we know, no HST). This leads me to believe it is a balance issue.
Larger items can be loaded in the rear of a freighter than in the front area (the ceiling is higher), so it makes sense to me that no HST is offered.