There's a term I once came across, the 'petrol-dollar' in some documentary whose title I forget. The minute Gadaffi, and Saddam before him, began demanding oil payments in €, their stay in power became untenable. It's instructive that the two countries that led the attacks, the U.S. and UK do not use € as their legal tender and the first order of business of the 'coalition forces' on landing in Baghdad was to secure the oil ministry even as other offices and ministries were looted and burnt.
That may however not be the full story but it certainly is a major part of it.