Because one of the great viscerally satisfying part of watching a sumo wrestling match is the strong initial clash. Side-stepping the charge can be a thing that makes you more likely to win the match, but it makes the sport of sumo wrestling less fun to watch and less prestigious/important/etc.

Basically, it's defecting in an iterated prisoner's dilemma. "Considered beneath the dignity" is just shorthand for "there are social enforcement mechanisms aimed at preventing this behavior".

The move is not described very well in the article. It's not really a countermove per se (because the time to impact is maybe 2 or 3/10ths of a second), but something that the wrestler decided to do before the match began. Before the initial charge, the two are crouched a few feet apart from each other, and instead of charging forward, one wrestler jumps to the side rather than engage the opponent.

I can't immediately think of any corollaries in other sports, but it'd be a bit like a baseball batter jumping into the middle of the plate in order to get hit so that he can 'win' against the pitcher.

It is a great counter, but sumo is a sport where acceptable behavior, inside and outside of the ring, is bound by strict rules.