Because it’s in everyone’s best interest that people with overt bias are dismissed. In high profile cases it’s standard practice for both sides to do pretty intensive research on individual prospective jurors (they get a list), and that often includes scouring the web for their social media accounts. If they find something you posted, and you didn’t disclose your account when asked, you could be in trouble.
I don’t think it’s usually standard to ask specifically about social media accounts, at least in normal mundane cases, but in a crazy case like this, it can say a lot about a person’s ability to be impartial.