I don't have a dog in this hunt, but here is my rationale.

Spirit, Frontier, and Allegiant comprise the bulk of the active airline threads in the Majors overall folder. (Not counting all the 'what are we going to do with NAI' threads.) They each are larger than at least one of the airlines that already has a sub folder.

Therefore, it makes sense to me to establish a sub folder for each of them.

Just like a filing cabinet drawer, put the bulk of the items in sub folders. Let the remainder 'one here and one there' remain as miscellaneous.

My two cents. Thanks for asking.