Trump is the one that keeps bringing this up. Why doesn’t he file suit? Could it be his legal counsel has told him to forget it?
I think Trump actually would like to have Ted Cruz on the ticket as his VP candidate, but he can't risk doing that as long as the issue hangs over America and the validity of the candidacy.
On that basis, it is possible the Trump lawyers are telling him to lay off the issue.
You can guarantee the Democrats are ready-and-waiting on the citizenship issue. But, for sure, Trump won't select him if it's not settled legally.