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.
If Syria were known to have the WMD, the pressure would be on the U.S. to eliminate the WMD in Syria. That is one more war than the Bush Administration wants to fight. In addition, such disclosure would put Israel in a difficult spot. The Israeli government would feel pressure to eliminate these weapons in the hands of such an ardent foe. That is a war that Israel probably doesn't want to fight now. I can see motivation for everyone involved to keep this quiet.