I think part of the reason is having an cross cultural insight to how the other person perceives flow. I mean, we can come from different cultures and each culture has their own manner of communicating disagreement from subtle to overt ways and it's expounded by the change in vocabulary. And sometimes people just don't want to put more energy on areas they can already agree or disagree on.

To be fully invested in a discussion and expecting the other to reciprocate that attention given online is just common courtesy but it happening online also means it may be asking too much. From both sides of the coin I can weigh in and excuse people for bailing out on the discussion no matter how invested I am into it, just cause they probably don't have the time to spare to dwell on it and that's perfectly natural.

Building and engaging is like a joke, it can be hit or miss and ones best bet is appealing to the general areas where mutual agreement can be reached rather than find resolution to a conflict that they may not be invested in the discussion at hand.