No, that's not it. Winterboard is compatible, it's just that not that many developers have created tweaks and themes (that work with Winterboard) for the iPad. It's like Winterboard is standing there, saying "I'm open, throw me the ball!" - but no one is throwing the ball... :)
For all but the past year, it's been all iPhone, all the time. The iPad community is playing catch-up (if they are playing at all...).