“The question is, why would the Administration want a secret backchannel communications link with Putin when he can go through existing channels?”

Have you been paying attention to the leaky sieve that Foggy Bottom and the Intel Establishment have become, especially when aimed at Trump? Had you been paying attention, it would be no surprise why back-channel communications (which are de rigour, btw) are attractive. You cannot be that naive.

Find something before the election, and then you would have something.