I'm just curious why Shimano hasn't come out with one. What are the technical challenged to doing so? I have seen the Di2 hacks online for mountain groups but Di2 has been out since 2008 or so and has trickled down to Ultegra but still no mountain.
P.S. I will not go create an account on mtbr just to ask this question so if you're about to complain that this isn't related to road keep in mind no one forced you to click on the link that *clearly* states what the question is going to be.
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Thread: Why no mountain Di2 groups?