Alta probably got a pass because it was American made and once allowed it wouldn't really be fair to not allow others. If American companies were still in the game I don't think the AMA would have been as easily pushed around by the Japanese and other ICE companies.
SX and MX under AMA Pro racing are dependent upon the OEM's paying for everything, so when they say jump, Feld and MX Sports ask how high. Until they have competitive competitors to the Stark, the door will remain closed, other than exhibition rides. Getting their ass kicked in the showrooms will be the thing that forces change, like always.
