Because if Vlad tells the world what he did, the next terrorists who take over a theater will use that information to figure out a away to beat it or control its effectiveness.
During the cold war the Russsians did a lot of research on gasses to use in all sorts of situations. They most certainly focused on gases that a normal gas mask would not stop. I doubt if anyone in the world besides the Russians knows if they were successful. If they were succesful, they are the only ones that know if all such gases they developed are lethal. Perhaps some are just disabling with the side effect of killing some who inhale. If such a gas were used, Putin would be stupid to reveal what it is. If that was not what was used, he would be stupid to reveal Russia would not use or does not have such gases. If all it was was a mist of valium he would be stupid to reveal that too. How stupid would Vlad have to be to tell terroists what he used and how he used it to defeat them? I submit he would have to be pretty stupid.
Vlad is not stupid. I will not comment on who is.