A possible explanation indeed, although in the facts it was probably in effect mixed up with blunders and simple lack of interest from the US: not enough oil in the Balkans. Kicking Russia out didn't appear then as the priority it is now, Russia was crumbling and nobody was envisioning the Putin resurrection to happen so quickly.

However, there is another component of US opportunism that played a part: after the fighting begun on the ground, the EU wanted to do something about it (some sort of Balkan angst, after all it's basically the backyard), and yet remained mired in a mess of century old alliances, hesitations, failure to commit adequate resources for a real intervention. Basically, they couldn't do anything without the US leadership, and for this very reason the US restrained from any decisive action until the self-inflicted damage to the EU was such that a common military policy became totally unthinkable for many years to come. The damage to the balkans in the meantime was a negligible collateral for Washington. Because at that time, a stronger Europe around a reunited Germany was the most obvious challenger they could think of, not Putin.

Pierre