Madagascar's economy is focused on vanilla, while Saudi Arabia's economy is centered around oil. Why have these countries chosen to specialize in these particular products?

Are there any options?

I see some aspects in common:

both oil and vanilla are relatively expensive and the countries can be sure to sell them. There isn't much competition (there is quite a lot of competition in oil, but the Earth needs a lot of oil) and they can be sure that the prizes won't be suddenly lowered.

Also, Saudi Arabia can barely export anything else, since it's mostly a desert.