One explanation mentioned in the commentary is that sounds produced at very close articulation points may clash. That suggestion should not be treated as a complete and universally valid formula.
Some close sounds combine easily, and certain sequences with more distant places of articulation can also feel awkward. Hence proximity of articulation alone cannot settle the question.
The practical task is to assess the entire pronounced word in light of reliable linguistic usage, rather than computing fasaha from a simple count of neighboring articulation points.