They were all previously EFTA members with solid economies that would helped to counterbalance the expected intake of weaker economies from expansion in central Europe. Scotland doesn't even have an (independent) economy.
Of course Scotland doesn't have an independent economy, it's part of a national union with 3 other countries. However, Scotland would have an independent economy if it was an independent nation. And that economy - based on Scotland's share of the total UK economy - would make it a very viable independent nation. Certainly a "safer bet" then the last 3 rounds of EU expansion.