It would not surprise me that there is general agreement amongst the highest level of the gubermint, both parties, that the real answer would scare the bejeebus out of the world and therefore must remain a deepest, darkest secret. It probably has something to do with international blackmail and sets of missiles retargeted in a kind of doomsday scenario. Only explanation that fits.
1) Iraq had WMD's.
2) Putin helped Saddam remove them to Syria before the invasion.
3) Bush took the "No WMD's in Iraq" domestic political hit to defend Putin's image, fearing the consequences of having the Russian nuclear arsenal fall into the hands of a Zhirinovsky-type fascist would be much worse.
4) No one thinks Syria is dumb enough to use Iraq's WMD's (mostly chemical warheads) in any way - but having them makes Bashar Assad feel like a "player", so everyone has agreed to leave him to his delusions.