Why? An instruction to stand behind the yellow line should suffice whether that be via posters on stations, PA announcements or directly by staff, as long as it's done politely.
Because stations haven’t had them for most people’s lives. They are fairly new introduction. There is lots of scope for a really nice, positive campaign to say, look at what we have done, this is why we have done it, remind people of what can go wrong by reference to real incidents, make people realise this is an act of protection etc. much much better than issuing barking orders to platform staff who seem on some cases to use the opportunity for personal exorcisms.
They haven’t been done well in many locations, leaving insufficient room for passengers to safely wait/pass. Those locations need a rethink.
For instance at Gatwick there is simply insufficient space behind the yellow line for passengers. So you have two options, contribute to a mass and dangerous bunch up at the bottom of the escalators that then turns into a mass brawl to get on the doors of the trains in those locations.
Alternatively, walk down the area over the yellow line and get barked at.
Every single train stops at Gatwick and they all seem to approach slowly anyway because of the quantity of passengers, so the yellow line could be much closer to the platform edge, more like the distance on the tube.