I'm guessing that part of the reason is that the plastic bobbins are much more readily available in places like Wal-Mart which would be where a person would most likely go first to find replacement bobbins.
Even as an experienced seamstress I did not initially recognize the differences between the various bobbins and attempted to use wrong ones on one of my machines and did not realize that was part of the cause of the birdnests I experienced with it. Need to go back and figure out which bobbin really was meant for that machine and see if that will make it work right. Also need a bobbin cover for it. The random piece of clear plastic does not substitute very well there either.