I know it will run afoul of the PC crowd, screaming stereotyping, but Obama is really lazy and a dilettante in the political realm. Consider ...
Building a coalition is very hard work and takes a long time. Instead of all that time on the golf course or on vacation, Obama should have been cultivating relationships with world leaders. If he did do work, it was in breaking down relationships - Britian and the Churchill bust fiasco a prime example.
Then, when it mattered most, he could not reach out to his fellow leaders and perhaps more importantly, their cadre of advisors, who would counsel their principals to engage.
Also, there was no trust that Obama would continue to do the right thing. None. His proposed actions were diametrically opposed to standard liberal pacifism not to mention his entire body of “work” as evidenced by past statements and votes. If countries spent political capital to join, would they be left holding the bag when Obama lost interest or got pushback from his base.
After all, look how he abandoned Iraq and is actively abandoning Afghanistan. And he can’t even keep his Gitmo promise - although that might be the one time where practicality won out over rhetoric.
Amateur hour, indeed!