It's actually a hard question to answer. Being a good place for innovation (as pg suggests) is definitely part of the answer. But if you look at a list of rich countries (excluding mineral wealth & havens eg Qatar,Norway & Luxemberg,Andorra), it's hard to put a finger on a cause. If you asked the residents of those countries why they're rich, you'd probably get few commonalities.

US: freedom, innovation

Singapore: extreme k-12 education, trade center

Switzerland: dunno

Netherlands: Progressive culture, social justice

Ireland: Intelligently designed & centralized tertiary education system, bribing key companies to set up shop

France: Liberte, egalite... ( just kidding, sort of)