Well - as a non-mechanical guy who can't assemble my own bike from components, I have to say that ... shopping for road bikes with Di2 or Campagnolo Super EPS is relatively easy - I've got 2 bikes with Di2 and one with Campy. When it comes to shopping for mt bikes with those components ... rarer than a hummingbird in Iceland. I've come across one or two in the past few years of online bike shopping.

Why it's not more widely used by manufacturers, no clue.