Problem is, people playing the game have a narrow view of the game. Developers on the other hand....
People may want X implemented. X may however require parts of the game engine to be completely re-written. So any options given out for voting will only really be narrow, proscribed options- in which case would it be worth it?
Equally it can unecessarily raise expectations. People voted for X, where is X? Why hasn't it been done yet?