Alex Massie assembles a theory:
Other countries, not merely in western europe, have relied upon US protection so heavily that they are now largely incapable of making large-scale interventions themselves. They need the Americans. One consequence of this is that when the Americans actually ask for help there is not much their allies can usefully offer. This strengthens the American view that the US is having to shoulder too much of the burden itself. There's something to this. But if that's the case then it's partly also because America's allies appreciate that the US will do what it must in order to safeguard its interests and so, even when our own self-interest might be aligned with theirs, the less risky option is to let them go ahead on their own since they're going to do it anyway. Without the Soviet Bear, Washington's ability to leverage support is actually more limited even as its own power has increased.