Because those alternative Girl Scout organizations were so successful. Then again, perhaps one can learn from their failures.
Biggest problem as I see it is that there will not be one Boy Scout alternate - a half-dozen will spring up, often fighting each other for funding, recognition, and youths more so than the group they were supposed to be supplanting.
What I am hoping will happen is that all of the troops that resign from the BSA will get together and form a conservative alternative.