Because treason requires an act of war against the United States or the giving of aid and comfort to an enemy. Espionage requires the transfer of state secrets to a foreign country. Neither apply in this situation.
Was the Hussein gambit an attempted coup?
And please don't tell me a coup has to be overt, kinetic activity. The founders, as highly educated Englishmen, were surely well versed on Macbeth, Richard III, Julius Caesar, et al. Richard, as regent to the two princes, simply had them disappeared in order to assume the throne. So, like the 1A doesn't mean just the printing press, and the 2A isn't restricted to muskets, a coup can involve subtleties such as espionage, subterfuge and legal maneuvering.
Once you accept the Hussein treachery was an attempt to overthrow the duly elected and legally constituted government of the USA, then you agree that it was a highly leverage revolutionary act of war. That means treason for all parties involved, not only those directly levying, but others providing subsequent aid and comfort.
Which, for those paying attention, would include Sessions if he is truly trying to oppose Trump's attempts at fully prosecuting the war effort against the deep state enemy.