Because it removes control away from players and makes everyone entirely dependent on multiple layers of RNG. If they wanted a fully equal opportunity system, adopt a token system so people aren't forced to not only run Ridorana several times, but run it several more when something like the OP depicted happens.
What concerns me is if they adopted this system in other content. Twelve help us all if Savage had Greed only rules. If you ever want to kill a pug scene...
