Too many PA rants will men people turn off listening to it. They will hear it and assume its just another rant about keeping luggage with you. If the PA was only used sparingly then people wold take notice more. Same with bleeping. people eventually ignore warning bleeps and miss the one that could warn them of a real risk.I don't think this was the issue here though
(I am not saying this was the case in this event - just saying)