I could be imagining this, but in Mr Roddenberry's creation of Star Trek, did he not have that everyone would go through the Academy and therefore be an officer?
I think that was his view by the TNG era, which may be why Wesley was made an "acting ensign" instead of just a "crewman." But we did see occasional enlisted personnel in TNG, although it took them a while to settle on whether Chief O'Brien was actually a chief petty officer or a lieutenant who just happened to be the transporter chief. He was still wearing lieutenant's pips long after he was established as an enlisted man.
In one of the tech specs books (for Constitution, Saladin, Hermes and Ptolemy classes) it gives department breakdowns and has no enlisted ranks, only Ensign and above.
That would've been Franz Joseph's Star Fleet Technical Manual
, which was not created by any of the actual show's creative staff, and which Roddenberry came to think poorly of later in life.