An NMO is essentially a sheet metal mount. An HF antenna, particularly lower frequencies are going to need to be loaded somehow. HF mobile antennas are generally base or center loaded. By this, I mean inductance is generally added to tune out the capacitive reactance of a shortened antenna.
Base loading can be accomplished with a relatively low (spell that small) value of inductance, but it's horribly inefficient. Center or top loading is is more efficient, but is far more demanding of the mounts mechanical strength, which is compromised, being on sheet metal.
So, it's conflicting requirements that lead manufacturers away from NMO mounts for HF.