Yes, it is all about costs, profitability, terrain and zoning. It easily costs $250 a linear foot in construction alone when figuring width of a site. Each foot has to have electrical wire, water line, sewer line, cable TV line, irrigation lines and landscaping. These lines have to be trenched in and the square footage of the site figured into the irrigation needs of the site. Each foot of site width adds a foot of road. So you are looking at $2500 in construction costs alone to make a site 10 feet wider. If the site and road together are 100 feet long, that's 1000 square feet of additional land per site. That means a 100 site park would be a minimum of 2.5 acres larger (and probably much more due to terrain) and cost an additional $250,000 to build. And that is only getting you an additional 10 feet.
Then you would have all the recurring costs of irrigating, mowing and maintaining an additional 2.5 acres. On top of that, those two and one half acres of land could cost upwards of several hundred thousand per acre, depending up where the park is located.
All this is the bare minimum of costs based on constructing a new park. Those costs would soar if it were upgrading an existing park since the old sites would have to be removed along with the existing infrastructure before the park could build the new sites.