There was discussion about whether to make the "Strategic Pool" available to all regions... in some other cases (such as the Space Race), we did do this, and allowed third-party sandbox regions to "win" or otherwise take advantage of the feature.
But for the Strategic Pool it just made more sense to limit it to the historical superpowers. While there are some very occasional examples of Silos outside of US/USSR, these were never anything substantial, and mobile land/air/sub launchers remain the platforms of choice for the rest of the world.