Ah, yep I didn't realize it was by invite; the TSA website confirms the participating airlines have to invite you.
MCO definitely left me wishing there had been a TPA-CUN direct flight; we drove out there to fly direct, the parking was expensive and 15 minutes from the terminal and security was pretty slow even for a Saturday. Glad I didn't jump to the pre-check line now that I know we weren't eligible lol.