Why isn't the pain constant with tn?

Lucky it's not. Good question. I don't think there is a simple answer for this. Possibly the irritated trigeminal nerve fatigues or is suppressed by the brain. A similar situation might be epilepsy where the damaged area only gets triggered periodically. Most people with tn couldn't stand having the pain constantly.
Trigeminal Neuralgia. Tn is not a well understood ailment. Often it occurs without warning. The episodes of pain, often with short bursts and on one side of the the face, may happen frequently or be rare. Understanding of tn is ongoing and still is relatively a mystery.