I'm not sure if the anti-TV reason is all that true, as Hunter guest-starred on TV years after Trek (ex. "Freeway to Death," a 1967 episode of The Green Hornet--ironically, another fantasy series)
Although he appeared in several television series after starring in "The Cage," as far as I can tell they were only guest starring roles. This meant Hunter would have to make far less of a time commitment (freeing him up for feature film roles). It also meant he could pick and choose his material (according to the description, he was dissatisfied with much of television because the need to constantly churn out material resulted in plenty of crap; a series regular would be forced to perform this crap, but a guest star would not).
Follow-up: Something I should know, but cannot remember at present -- were scripts for the second pilot ["The Omega Glory," "Mudd's Women," and "Where No Man Has Gone Before" were commissioned before the latter was chosen] initially written with Pike instead of Kirk, or was the character recast before the episodes went to teleplay?