Its allways down to an individual club what can and cannot - but some of te other problems are that helpers simply dont have the experience of many breeds and a helper working with any of the very heavy dogs is really getting fatigued quite fast and loosing his abilties, they are also only trained for one or two breeds anyway.
The other thing is that for the world championships, which is what its all about anyway, there is no way many of the larger, less agile and slower reacting dogs would get anywhere-if the dogs dont then the club doesn't.
At the end of the day an agile breed which does not exhaust very quickley because of the weight it has to carry will work at otimum capability all day, few of the mastiff types have that stanima and mainatain speed and agility at the end of a day.
You will find in Europe they breed much lighter dogs for SchH than their working counterparts for the simple reason they get more points, which is what SchH is all about, but put the same dogs in real life working role and they are simply not strong or heavy enough and most would not have the character to train as far as a real life working personal protection dog.
Haveing said all that why dont a few enthusiasts of your breed start training as licenced helpers and start your own club or try and find a helper apart from that, good helpers are few and far between,if there are to many barriers with clubs, anyone can start a club if they want to, relying on someone elses club to change the rules they are satisfied with is a bit much?