Because no one has used Symbian in 10 years and the vast majority of us have never seen or heard of a .sis file? Not really parity if .sis is extraordinarily obscure and .bro becomes pervasive.
Don't speak for the entire world ("no one"), I personally know two guys that still use Symbian-powered smartphones and don't plan to change them to anything.
I know more people than that who still use a Commodore 64 on the regular. But, it doesn't make it a viable platform.