As I often do, I find myself here agreeing with Earl. Why no triples? "Just because." Also, because the more birds you add, the greater the tendency is to have tight, technical tests, which reward the trained, lined response.
I got my start in FT in derby in 1999 and remember traveling all across the country and competing other amateurs. Now, amateur handlers in the derby are few and far between. I wish that clubs devoted more resources - judges, grounds, and help to the derby.