Are most of the pictures you googled from Texas? LOL! There are a few barns around my area (central TX) that show in all of the same classes and are different breeds, but they all seem to band together and host their own shows separate from the stock types due to the fact that you can't fairly compete.
One of our 4H parents (and our 4H is predominantly stock horses) said that the judges told her that they don't know how to place any other breed that walks into the ring that isn't a qh type especially horses like arabs or warmbloods or anything else that might have more step to their movement than toes dragging the sand. Their movement is just different.
In halter, the judges here won't even look at you because they actually compare the horses to each other, and again if you don't have the stock horse build, you can't/won't be placed.
Enough people protested that it actually came to a point now where for every show that is advertised, it is now part of the 'norm' to advertise that all classes will be judged according to AQHA rules/guidelines and all conformation classes, specifically halter, will be judged according to stock horse types. Again, I'm just speaking from my neighborhood.
All they did was succeed in segregating everyone and no one goes to any of the western horse shows around here unless they are entering a stock horse.
A friend of mine shows quarter horses and english with her arab and she goes crazy trying to do two shows at once on the same weekend - her qh's with the other stock types, and then trailers her arab to a completely different barn to compete in all of the same classes with other Arabs and some other breeds (the other shows are VERY friendly towards all breeds participating).
I think it's just because TX is quarter horse central on the whole planet. And when you have enough of one type competing on a regular basis, there's enough of you to start making your own rules. Lord knows we have plenty of stock horses here to go around.