Because that's the way it was programmed. You can e-file today, or tomorrow, or wait until October 15.

{edit} And you will owe interest and penalties, too.

The payment was due back on July 15th, so since its late, it will only let you choose the current date for direct debit.  

If they don't want to pay now but you want to Efile and get it off your desk, they can make payment when they're ready at IRS Direct Pay, or give them a payment voucher to mail a check.

I don't remember not being able file a return with a payment date later than the efile date.  Has it always been like this?

Is this what you didn't understand: If there is a payment owed with the return, which is on extension, you should already have scheduled and made the payment when the extension was filed for. Filing the actual return now means the payment is already late. You are being asked to schedule the payment that is already past due. It isn't due in Oct. Only the form(s) have the Oct due date.

You can't schedule a direct debit after the due date. 

If you had E-filed in June and scheduled the debit for July 15th, that would have worked.