[lurker]

ffr didn't allow for custom keys so play was focused around the dpad
one hand and "four key" (proto spread, most people's keyboards didn't allow hands) were natural choices for those with little to no DDR experience. those who did play DDR, however, played index since that more accurately simulated pad play
in the early years index was the fastest way to play since it gave one's fingers the most freedom to move around
stepmania allowed spread play years before ffr did so it's no surprise innovations happened there then very slowly trickled down