Because it is a classic case of its not that plain and simple!
1st off, you need to have solidarity, at these supplementals getting all the people on the same page is not easy, because where there are less than say 100 pilots, everyone is on a first name basis. When you throw in management types in there it starts to get difficult for those with initiative, see above. get in some people that got in via a pft issue and they could care less, since they are right out of school and already in the flight levels and they dont want to jeopardize "their investment".
Since upgrades arent based solely on seniority at some places, those that are "next" do not want to do anything to offset their standing in management's eyes. Finally, when you do get some cohesiveness, a majority, it is usually weak and stands to face its originators being fired for some reason, we all know what that is, and then someone else has to step up. Then that someone else now has to go to ALPA only to find out that they will only talk to you once you have a minimum number of pilots to make it worth their while. Your other options are teamsters, which have their own minimums, and then you are not the first on their plate. Meanwhile the months go on, attritions continues and your support goes with it.

It is an all the way uphill battle that only people that care long enough to stay around will initiate. it is freaking tough!!! Im not saying it cant be done, as there are many supplementals that have their own independant unions, but for say a focus or cargo 360 that just started up.....you are basically the guinea pigs for everyone else to gauge whether or not promises are lived up to or its even a place where youd want to go.

im sure ive missed other points, but those are some of what the nonskeds deal with, so back to the original point, its a classic case of its not that simple!