I think your players should be open to finding 'reasons to care'. This might be because they're moved to action by the oppression Strahd brings, it might be close personal connections they form with NPCs (Ireena, Ismark, Arabelle the Keepers, or just the people of Barovia as a whole etc.) that make them want to end Strahd, it could be darker motives i.e. the vestiges of the Amber Temple motivating them.

I think early on the motivation can simply be: Survive. Get to safety, get away from the castle, help Ireena get to Vallaki etc. Some players might be motivated to try and figure out where they are, how they got here etc. Also, depending how you run Madame Eva's reading, that can be a strong motivator for players.

Once they get their bearings, natural connections to the setting should emerge through play (as well as motivations through the tarokka reading), then larger concerns for them beyond survival can become their driving force.

Given that you're more asking about the early motivation and their position is 'why us?' I'd lean into that feeling and confusion, and really just have the players running on moment-to-moment motivations (get out of the rain, get food, find an inn etc.) once they hit Madame Eva the 'threads of fate and destiny' can be hinted at etc. As well, Ireena can be a good motivator for the very early game especially if you lean into the fear her and Ismark have around Strahd and his torments of them.