Because there are thousands upon thousands of threats from Muslims, many of which are valid.
Sorry, no excuse.
Either we are at war with isis, or we are treating terrorism as we did during the Bill Clinton years, that is, as a criminal issue.
If they knew they were monitoring isis terrorists and thought there was a credible enough threat to warn Barcellona, there is NO excuse why they just didn’t get them on their own.
Many times through the years I’ve either read about or heard of someone who made what could be considered a threat on social media, and the Secret Service was at their door in a matter of hours.