You can't police every doorway and cyclists will often cram on wherever they can.
Guard's vans, again, or the lack of, and in more ways than just that they aren't there.
Boarding a train with a bike is difficult. You have to spot where you're supposed to get on as soon as possible after seeing the train approaching, so that you can get to the right spot on the platform in those last few seconds before all the people start to move forward and there is no longer room to push the bike along the platform. Otherwise you're pretty well stuck until most people have got on and most people getting off have dispersed, which means you're holding things up and risking having the train go without you.
With guard's vans this was fairly easy. The distinctive layout and shape of the doors and windows was easy to spot even from a considerable distance and at an awkward angle, so you could work out where you needed to be well before the train reached the platform, and there was the further advantage that that was the one spot that would
not attract a ruck of people trying to get on, so there was no conflict for the space.
These days it's a pain. There is no chance at all of being able to work out where the bicycle accommodation is from a distance. You have to hang around at the back of the platform, since that is the only place where there might still be room to move once people start preparing to board, and then try and peer through the gaps between all the bodies and into the windows of the train in the hope of seeing where the interior layout is different - an endeavour at which the chances of success are pretty low unless you're the tallest person on the platform. Bicycle symbols on the side of the train are useless even if they're there, because you can't see through the people. Even if you do manage to spot where the cycle space is, it may well still be impossible to get to it - for instance if you are between the cycle space and the station exit, so the space behind the crowds trying to get on the train is filled with another crowd of people all walking the opposite way to you.
This makes it very easy to end up in the situation where you have a choice of trying to cram on at the nearest door regardless, or being left behind because you can't get to the "cycle door" in time.
Furthermore the current situation engenders the perception that it doesn't matter anyway, the so-called cycle space being so totally unfit for purpose that there's no point worrying about trying to use it. If it has tip-up seats they will probably have people sat in them so you can't use it anyway. Or the utterly stupid cubby-hole I was once instructed to put my bike in which was too narrow for the handlebars so the front half of the bike was sticking out right in front of a doorway - if it hadn't been for the guard telling me I wouldn't even have thought that that was cycle space, let alone tried to use it as such.
So "cram on wherever you can" is by and large a valid strategy, and the reason for this is not within passengers' control.