The Habsburgs gained a lot of their lands through marriage to the point of having an empire where the sun never set. I can imagine that one of the reasons why they didn't marry the Hohenzollerns could be that they were protestant, I'm not sure if the Habsburgs have any protestant members of their family( I doubt it, but you never know), but once Prussia became a considerable power, wouldn't it have been in the Habsburg interest to bring Prussia into the fold through marriage. Did they ever forgive Prussia for Silesian episodes?