Yeah, I am curious as well.
As an ASA guy, I will bet there is no single reason, but it is likely a range of things:
-Management relations have been just good enough to make a union not needed badly enough to go through the trouble. They are like that one kid in the trailer park who has never been beat when Mom runs out of crystal. They just don't know why the other kids are always calling the cops.
-They have enjoyed the "rising tide raises all boats" effect of union based efforts everywhere else in the industry. They get the benefits without paying the dues, because the company sorely wants to keep ALPA off the property.
-Union dues. In their situation, it can seem an unjustified pay cut.
-They have never been educated as to the industry level good things ALPA and other unions have done which benefit the profession as a whole, beyond pay and QOL. Safety being the biggest item here, from TCAS, to GPWS, to ASAP. And ALPA medical assistance is worth every dime of dues when the dookie hits the fan on your next Class-I.
-ALPA comes across as a bunch of Guido-esque used car salesman.
-Jimmy Hoffa type affiliation.
-And did I mention the union dues?
-The pro-union drivel on boards like this is usually not well played.
-Prater looks like Jabba The Hutt.