Because the statements and allegations made by the petitioner are judically privledged. I know this doesn't seem fair, but anyone can say anything in a document filed with the Court and those statements are protected. The theory is that if they are false there is no harm to you, as no order will be entered, but if there is a chance they are true, it is critical for persons to be able to communicate with law enforcement and with courts without recrimination.
Either there is a reason to state a claim against the other person, or there is not. Frequently, the basis of the claim is that the other party filed for an anti-harassment order, and for no other reason. That isn't a sufficient basis to state a claim.
I know this isn't what you wanted to hear, but I hope it helps. Elizabeth Powell