A lot of this was covered in the recent MRL Trains article but some of it was not.
MRL was spun off as an excess line by BN because of it’s steep grades, light online traffic and the fact that BN did not need the extra capacity.
MRL insisted in it’s operating agreement that BN would contribute X amount of overhead traffic to keep the line financially viable. so far to date BN and BNSF has not hit that minimum.
MRL’s online traffic seems to be pretty sparse from the article but earlier posters are correct in that the line being kept open is important to Montana farmers and politicians just as maintaining the ex-Milwaukee Mainline to Montana is important. Pretty sure they had a say in the agreement as well.