I agree that it's a shame access to this land is now in serious doubt, but I have a different "take" on it...
Using land that is privately owned, or owned by federal or state governments, is a "privilege". It is not a "right". To keep that privilege we, as users (whether our activities are hunting, fishing, hiking, snowmobiling, etc - it doesn't matter) must act responsibly. Not doing so puts that use in jeopardy. There are several well funded environmental groups trying to restrict access, and not respecting land plays into their hands.
Several times a year I go to southern Utah and use a Polaris RZR to explore Sand Hollow State Park, as well as a huge amount of contiguous BLM land. It disgusts me to see the amount of trash people leave behind.
I'm not jumping to the conclusion that this happened with the land the Wilks purchased, but if I owned a big chunk of land land and graciously allowed people to use it, and they then made a bunch of new trails and left their trash... I'd close it too.
As others have pointed out there are possible legal issues with allowing ongoing access, as well as potential legal liability if someone is hurt on their land. So why should they open themselves up to that liability?
The Wilks brothers rose from humble beginnings. I suspect they're relatively normal guys and my guess is if one were able to speak directly to either of the Wilks brothers and asked nicely, they'd probably give you permission.