Was it always a straightforward presumption ("davar pashut") that in general the Ashkanazim should decide Halachic questions ("pasken") in accordance with the Rema over the Machaber/Beis Yosef (when they disagree)? Or did one of the Achronim come along and say that in general the Ashkanazim would/should pasken in accordance with the Rema over the Machaber, but before that particular Achron it was not necessarily for an Ashkanazi posek to rule thus?
Actually, it's the other way around:
The Bet Yosef wrote the Shulchan Aruch (as a summary of the Tur).
The Rema noticed that it was comprehensive, but went against some Ashkenazi accepted Psak. So he decided to add those in-line.
Result: Ashkenaz Psak is like the Rema and Sefardi Psak is like the Bet Yosef.
It's not a presumption, it was purposely written that way.