Probably because they have caught all kinds of H. E. Double L. from people who had to wait for their site to vacate while other people just breezed in because the site they had reserved was vacated. Those that had to wait probably made life for the admission attendants miserable asking why they couldn't just take the first empty site and the person who reserved that site couldn't just take next empty site.
Also, depending on the situation, they may also have had complaints from the people who were on site and didn't have to leave until checkout time that the people who were going on to that site next were circling them like buzzards making them feel uncomfortable that they weren't rushing to leave before checkout time.
And there also probably were people who truly were abusing the fact that they didn't enforce check in times. Things like people arriving at 8 AM for the 4 PM check-in. Those early check-ins might interfere with site maintenance and the like. Personally, if check in time is 4 PM I wouldn't be upset if I couldn't get in at 2 PM. That is why they post the check in time.
