Why are people not opening new campgrounds/RV parks?

It's only a good investment if you can charge enough to people who are camping to cover the costs of running the campground and the capital expenses of building it, amoritized over some reasonable period, and still make a reasonable profit. That's very often an iffy proposition at best in practice between real estate costs, zoning and environmental regulations, payroll, etc. etc.; and frequently there are other businesses or uses for land that offer a better return with less risk for the investors.