[Well, that sounds like an excuse for him to ramble about what he did, so guess what? He's going to ramble about his evil plan now.]

Because I destroyed all other forms of currency or rendered them useless. It was quite brilliant really. I treated them with a chemical compound that would eventually ignite when triggered by a specific signal. And in exchange for the tainted currency, we provided the world with unaltered Cobra currency to use for all future exchanges, effectively returning the entire world to not just a gold sssstandard, but a Cobra one!

[And now there's the customary evil cackling at the end of the story.]