Her father was a lord. Things were much different in Winterfell than they are in King's Landing. For the most part, she's been cloistered away (aka held hostage) during most of her time in King's Landing, so she really hasn't had much contact with reality.They definitely don't let her walk the streets of the city. Because she was relatively isolated up in Winterfell, she was allowed to not think about harsh realities. Arya was much more interested in the real world. Sansa wanted to embroider pretty things and marry her prince.
Plus, we know that she thinks Joffrey is a monster, but it was probably a shock to learn than lots of common people feel the same. I think when you live as the betrothed of the king and have servants at your beck and call it is a shock when those horrible harsh realities intrude. She's prone to being all airy fairy anyway and she's definitely not used to taking care of herself. Tyrion and the Hound notwithstanding, she doesn't really have a protector. I think her rude awakening is happening in stages.
I enjoyed Tyrion slapping Joffrey in this last episode just as much as I enjoyed the multiple slaps in season 1. IMO, Joffrey really can't be slapped enough.