Plenty of reasons why certain countries decided to stay out of the coalition against terror, but this is a big part of the big reason: fear. They didn't want their people targeted.
Little do they know that they are targets anyway, and the favorite targets of terrorists: people who don't fight back. The Mighty DF has already indexed a wealth of reports of Islamic crime in France and elsewhere. The leaders of the timid shrug, and cluck their tongues at Spain and say, "this is what happens, I told you so," and laugh nervously when the bully turns and hits them instead. "Why can't you follow our example and be polite little stationary targets?"
Nuts to that. Spain is right to fight; better that their soldiers risk death to save their people, than to sit idly like a lobster in a pot while it slowly rolls to a boil. Maybe it doesn't mean that much, but the US will not forget who has stood with them in this fight, and they will stand by Spain in the same way. God Bless and comfort the bereaved and the dead. Now let's stomp some terrorists.