Because the campaign isn't about how to respond to an emergency? It's come about because people are scared of reporting things they see as being small, inconsequential, non-life threating or not an obvious emergency. Someone hanging around staff doorways trying to sneak in - not an emergency. Someone talking photographs of CCTV cameras - not an emergency. Etc. Etc.
It amazes me that people want the message gone because they're fed up of hearing it, but then don't actually know what the message is actually saying, in plain English!
If you see something that appears suspicious, if it doesn't look right to you, whilst on the railway, you don't call the emergency services, you notify the people best placed to make a decision on how serious the threat is...the BTP...on 61016. It's all there.
The frequency, tone and delivery are another conversation entirely. But the point of it's existence, how is that still a topic of debate??