I've seen your other question when you asked "What does it mean if a women is "classy" i didn't answered, cause i didn't know how to explain or how to start & finish the point!
But now i get it why you asked that & it guided me to replay you about it.
There's no why. Because there's no rule for being classy. In all the respect i have for Miss.Hathaway, i like her acting & i watched so many of her movies. But the "Classy" expression is only matter of opinion. There's no basic qualification of being classy, Being "Classy" is a just a positive moral. Someone i call classy & they're not classy to someone else. That's just as simple as that.
-Maybe you seeing huge of people calling her classy. (well surely they are her supporters) if you face someone that don't like her, you wouldn't hear this anymore.
-Maybe you seen she's been called classy by some directors or judges or i don't know in some particular events ? (directors & judges has their opinion so, it's yet a personal opinion, for instance, Christopher Nolan as a screenwriter & director, he likes her enough that usually he would pick her for the movies he's involved. So to him surely she's classy! But it might not be classy to so many others)
In the same time, everyone has their own definition of being classy so the choices completely become different. Therefore, there's no sureness to "Explain" "why" Anna Hathaway is classy in compare of others! She might be that to some, & she might not be that to so many others as well. It's just all about personal preference.