What if you go 1-0 down in a game and a drastic tactical change is needed? Everybody huddle up on the sidelines and vote? What if it was a tie, who decides on the deciding vote etc. Also the guy on the sidelines would feel pretty worthless if people were making his/her decisions. It would be a good idea for picking the starting line-up, as the fans know what is best most of the time, but for in game tactical decisions it would be a shambles.
There are probably going to be some clubs who try for joint managers again, but it's been shown to be a mistake in the past.