poly silicon
poly can be heated in very high temp. will metal "as Al" will propably diffuse through silicon "i think this is the main reason", poly can be doped and hence we can control its c/c's Vth .
poly is bad in terms of conductivity "smaller than metal", i think that we may return to metal eventually when the VLSI technology advances more and more.
Added after 1 minutes:
btw the diffusion of Al in silicon is bcause AL is soluble in Si "i think so is Cu"