From what I understand (not being a BlackBerry or Android developer, just a small iPhone one), programming for BlackBerry is more tedious and low level than programming for an iPhone or Android. The SDK is less user-friendly, and one has to develop for a lot of screen resolutions (similar to Android, not really so for the iPhone).
All in all, if someone wanted to create an app like that for BlackBerry, there shouldn't be anything stopping them, just that for now there doesn't seem to be anyone wanting to do so. Then again, since there is more and more emphasis on webapps for any smartphone, maybe the need for BlackBerry-specific app is not that urgent.