Why couldn't this motorhome been allowed in to wait in the late night arrivals area which was empty for 15 minutes 

Probably because by this time next year there will be 7 early arrivals starting from 11.15 as they find it convenient for themselves.

You could equally ask why the MH didn’t go down the road to wait in the first place instead of pushing his luck by turning up early.

I don't understand any of this, we have arrived early twice, and were allowed to book in and go to any pitch available.  Maybe we were just lucky.

What’s not to understand about “No arrivals before 1200” or whatever time is applicable to a particular site?

It’s stated for a reason. Why flout it?

There is early and too early perhaps. In this review the OP says that he arrived at the barrier at 11.45 and the motorhome was there then. We do not know that it was exactly 11.45 nor if the motorhome had been parked up 15 mins earlier. The site staff are geared up to accept new arrivals at 12 and often may not be available much before and may be called away from the office and so a number of early arrivals can quickly lead to traffic problems on the highway. 

On our last trip I read the comments book at one of the sites. There was a complaint that a toilet block was flooded and when the writer later went to report it the office was locked up outside of stated opening times. The wardens' comments was something like 'who do you think was sorting out the flood?'. So the wardens may not be available prior to arrival.

As an example of the sort of problem that early arrivals might cause if members started to think that it was acceptable to turn up early - We arrived at Broadway Caravan Site after 12.15 on Sat 11th May. A caravan had pulled up at the barrier just moments before us and our caravan. Naturally I stayed with the car so as to move forward when he moved off. I could have asked OH to go in the office whilst I stayed with the car but chose not too. Two motorhomes pulled in behind me and the driver of one and the couple in another walked past me leaving their outfits without driver. When the caravan in front moved onto site I drove forwards leaving a space behind me of some 12m by this time the motorhome behind me had moved off and so the space was about 20m but not only where the occupants of the next in line both in the office a third motorhome had actually just cleared the main road and both occupants passed my outfit before I could turn off engine and exit car. One of the wardens left the office to go and watch the road to direct any more arrivals past the blockage and clear the main road. This is an example of how thoughtless/selfish some folk can be. If these arrivals had been at a time before the wardens were available chaos could ensue. There are two toilet blocks on site and the cleaning of one is scheduled to be completed by 11.45am and so available staff may be fully occupied away from the site office.