Once upon a time that was a question of the century but couldn't imagine if someone would ask this again.
Yeah, stake and primedice have the same owner and there is a big story behind it. Both of these websites share game dice but the reason why primedice is standing dedicated is that this is one of the first bitcoin dice game that was created and because of this website's story and it's ancient feelings it activates, they decided to leave this website on it's own and create another one, called Stake.com (yeah, nice and expensive domain) and make Stake crypto casino while keeping primedice dedicated by running dice only.