iOS 5 requires a valid APTicket with the SHSH blobs. These AP tickets are time limited, so we weren't able to simply save them and replay them. A4 and earlier devices have the limera1n exploit, so the devs were able to get the processor to validate "old" APTickets.
A5 devices had no such exploit, so it was impossible to downgrade them. This new method uses some kind of trick to validate the old tickets, but it will likely be patched out soon.