FOLLOW THE MONEY: The Trump admin wanted a secret channel because the communications would be about money.  Various members of the Trump admin did not act on their own initiative, they communicated with Russians on the orders of the president.  Putin and the Oligarchs need their money to be laundered and out of Russia for when the Putin regime inevitably ends.  Real estate held by Trump, Kushner, and others is the perfect vehicle to park their billions.  This is the reason behind all the contacts between the Trump and Putin admins.  This is why Trump will never release his personal tax forms or those of various Trump companies.  This is of course speculation; however, it fits the Trump admin's bizarre Russian fetish.