The 'scheme' allows citizens of certain countries to enter the UK, as au pairs, without the usual visa restrictions which would apply. At one time, there were many European countries within the scheme but, as the EU has enlarged, many countries are now no longer part of the scheme simply because any EU citizen has the right to work in any EU country. (For example, Sweden isn't in the scheme because any Swedish national has an automatic right to work in the UK).
The au pair scheme has, to the best of my knowledge, only ever applied to nationals of countries within (or on the borders of) Europe.
Chris