I'm not so sure Tower of Terror is as popular as everyone makes it out to be... It is a ride that many people are, of course, to scared to get on... With wait times increasing mostly due to only one side running, and sort of a small capacity thing going on.
Meanwhile Rockin' next door has these super long trains going every 30 or so seconds and still has long wait times, because roller coaster. Tower gives the illusion of being this hugely popular ride simply because it is that, huge.
I'd be interested in knowing if that is why there isn't as much merch for Tower as there is, say, for Haunted Mansion (which has its own shop altogether)...
I'd like to point out, though... ToT is one of my favorite rides of all time. I only mention this because every time I go to Disney World with anybody, this is the MAIN ride that half or a couple of people in my group will NEVER ride.