For the same reason I get frustrated when I go to solve problem A, run into problem B then while trying to solve problem B I run into problem C. Soon enough you have 15 problems you want solved but you find yourself having to unwind a stack 1 by 1 until you _finally_ get to a point where you can actually solve the original problem.

People probably view dealing with the FF codebase as part of Problem A, dealing with mercurial is Problem B, which is must less appreciated.