NOTE: Like FumbleFingers, I'm more comfortable with sociable here, so I'm going to use that instead, but it makes no difference at all to what I'm saying. If you're interested, the matter has been discussed on ELU.
These two questions reflect somewhat different meanings of BE when used of persons.
When we say of someone that he is sociable (or gracious or funny or a complete jerk) we are speaking about their character, who they are.
But when we say of someone that he is being sociable we are speaking about their behavior, how they are for the time being acting.
Similarly, when you ask someone "Why aren't you more sociable?", with the simple-present verb, you are asking about the simple-present version: "What is the cause of your unsociable character?"
But when you ask "Why can't you (or won't you or don't you—they amount to pretty much the same thing) be more social" you aren't really asking a question, you are challenging their behavior: "You really ought to behave more sociably, and I can't understand why you don't."