The problem is out of those 745 only 47 are under 2500 portals (the initial value) which makes it roughly 6% of total user base (game center enabled user base to be exact). This begs the question: what causes such a low number? What comes to mind first is: few people bother completing those levels again - game is just boring to replay all over again. I see point in that.
There is just one thing that bugs me: there are currently two players that got better result in 'Least Portals' than me. That is correct. My best test run was 379 portals. They made it 370 and 373 respectively. Get Portaball and check for yourself ;). They've beaten game dev in his own game. This is quite fascinating! It makes me wonder how can I improve myself, where can I find those best paths to get better, even though I've completed each level tens of times. This means, that there is fun and thrill in playing Portaball again, though it's quite hard to discove. I guess it requires some kind of perfectionist nature, willingness to improve oneself in every possible way.
The other possibility of weak Game Center attendance is level of difficulty of Portaball. Is Portaball an easy game? I have absolutely no idea. Obviously, for me it's easy, I designed every single level. Test results on my friends introduced various feedback. Some found it relatively easy, they've burned over 40 levels in one hour. The other group struggled with some of the first levels. What might cause that is lack of step-by-step tutorial. On the other hand I've designed every introductory level with a self explaining puzzle. Right after starting the ball it shows how different elements behave. Player's only task is to get the idea. Some levels are more demanding. Level 4 is good example ('speedy thing goes in, speedy thing goes out'). However, once player understands that it's all about acceleration, it becomes trivial. Getting that acceleration thingy might be hard, however, I believe it's one of the crucial and most rewarding moment ('OH! So that's what it's about! I'm genius!')
To sum up: if you struggle with Portaball, keep playing. The satisfaction is well worth it. If you find Portaball to easy, give 'Least Portals' a try. Getting on top and competing with others, that's a real challenge.