I could be wrong here, but I'm pretty sure each version of PD was its own database.  Meaning that if Stunna/Ed wanted to let users switch between PD1/PD2/PD3, they'd have to have all the servers, bets, and databases for every single one just so users had that option, since users and their stats were wiped each time.

I don't know if there was a wipe from PD1 to PD2, but I know that accounts at least were fresh from PD2 to PD3, meaning none of them carried over.  If the same is true for the first two, then letting users switch "versions" would be a lot harder task than it seems.

Not trying to say they're not capable, I'm just saying that if PD3 alone has almost 11 Billion bets with thousands and thousands of accounts, if they wanted to actively use all that data with the previous two versions, they would need a LOT of equipment and human resources to manage it all.

Sure, PD has its issues now and then, but I could only imagine the workload if they had to manage all 3 versions at once.

By "other Primedice versions", you mean the Primedice when it was first released and looked like the following (pic found in page 7 of this thread)?
http://i.imgur.com/erNEXYb.png