Because Serj still contributes in many other ways and Daron was still a necessary piece of the puzzle and brought something to the table that made the record better than if it was just Serj singing (in my godly, sexier, manlier, more powerful opinion); regarding his vocals, yes he had to share the mic with Daron much more but it isn't as if he is not heard on the album, there's still plenty of Serj. To be honest, I don't love Serj's vocals anyway so I personally didn't give half a shit; that's not to say I dislike them in any way, his vocals fit the music they create very well, it's just that it's not a voice I'd like to hear in any other context. It just so happens I like Daron's crazy all-over-the-place vocals for SOAD's music more; and both of them combined is even better, it's perfect.
I don't get the beef, but then again you and I are clearly on opposite sides of the fence. You don't like the music cause of Daron and I happen to like it more because of it. The hierarchy of who gets the most time on the mic and what responsibilities were handed off and blah fuck blah don't really matter to me. The music is badass and it's nice to masturbate to whilst a belt is tied around my neck and my hermit crab watches, that's all I care about. Perfect music for that. Especially Vicinity of Obscenity. NOW GET OFF MY LAWN! AND NEXT TIME I SEE YOUR TOMATOES GROWING NEXT TO MY CABBAGE THERE'S GONNA BE A WAR. OF VEG-E-TA-BLES.