The Hulk's express leads you to the 15 min. wait mark of the regular line. Rockit's express is an entire line that leads you to the front. The problem with Rockit is that there's 12 seats in each train, compared to the 32-36 seats on the Hulk. If Rockit was to get Express, the line for the regular line would be longer than usual, and the single rider's line would be too unbearable to use. I remember on the single rider's line that I had to wait 15 at the front of the line just to ride Rockit. There are a lot of groups of 2 and 4 from the regular line that fills up the seats easily, and the wait is still long. When you add Express, the team member has to switch back and forth from regular to Express, and will causes a longer wait.
Also, keep in mind that Rockit uses a moving walkway to load the guests, just like Forbidden Journey. On Potter, the lack of express and single riders = a smoother and more consistent flow of capacity. Even though Rockit has this luxary, it's many breakdowns and ill-manufactured trains causes an inconsistent flow of capacity.
Hey, I love Rockit, but getting on the ride is a pain in the butt.