There are probably several reasons - some have been mentioned already.
For one thing most of the "systems" are loose methodologies and most "methods" are loose approaches. Therefore, results are going to be largely trader-based.
Another main reason, IMO, is that anything resembling a challenge to a method's legimacy is taken, by it's practitioners, as an attack upon it's guru/founder . Then all objectivity goes out the window and the fireworks really start. (Note: check back on my posts - I've been on the embarassing side of that equation)