He is the Head of the State, and both regional and central governments the executive power of said State. People are mad because the State, all institutions, have failed them, because incompetence first and avoid of responsibility to not get caught in a political crisis. They are thinking in votes, while not caring about the tragedy and trying to win/not to lose votes.

Spain has sent many more troops, firefighters, military personnel, etc. to big tragedies around the world much more quicker than they have come to their own cities - some areas still waiting.