It's the way the new iOS 5 has been designed. It changes the baseband of the 3G iPad once you upgrade to iOS 5. So, once you are on that version, you can never go back down, even with SHSH blobs.
However, because the WiFi-only iPads do not have a baseband, this "problem" doesn't affect them.
I don't understand much more of the technical details/reasons. Our resident jail breaking expert, f4780y, can give you all the specifics, plus he's written about it before (I just can't find his post to quote it!).
Suffice it to say, if you have a 3G iPad, do NOT upgrade off of 4.3.3, or you will forevermore lose your ability to downgrade, SHSH blobs or no
Marilyn