because traditionally the male was the wage earner. so whoever he married, his standard of living doesn't change. in contrast, in traditional (ie my parent's generation, uk) roles, who the woman marries fixes her standard of living.
In modern life and in the US, we're much less aware of social class at all (or actively dislike the concept), and it's gotten (somewhat) decoupled from economic class. Someone who has become a lawyer or otherwise economically successful could be from any class and it's not unlikely they might choose to marry someone from a similar social class (with similar background) rather than someone with similar economic class.