I maintain that it was primarily timing and logistics.
The Ents were busy defending Rohan very shortly before the Battle of the Pelennor, and things happened so quickly that they weren't able to be of any assistance:
March 11 Eastern Rohan is invaded from the North.
March 12 The Ents defeat the invaders of Rohan.
March 13 The Pelennor is overrun.
March 15 Battle of the Pelennor.
Selected excerpts from Return of the King Appendix B "The Tale of Years" (ii) The Third Age The Great Years
Consider the map below, and bear in mind that it took Théoden and company six days to march from Dunharrow (approximately the blue circle) to Minas Tirith (the red circle). Admittedly Théoden's army is much bigger than the Ent army, and the Ents can take much larger steps, but factoring in the time it would take to send them a message and it seems unlikely they could get from the northern borders of Rohan to Minas Tirith in time enough to be useful.
Don't forget that it took them a night to get from Isengard (the green circle) to Helm's Deep (the orange). It's much farther to Minas Tirith; they could probably make the trip in three days, but not when you factor in a messenger.
Of course, if I were Théoden and leading every able-bodied man into war in Gondor, I'd want to leave behind a rearguard to keep enterprising Orcs from torching my villages. It's entirely possible (though unconfirmed so far as I know) that he didn't want them at the Pelennor, because they were much more valuable to him defending Rohan while the entire Rohirrim were doing other things.