There are many advantages of allowing mountain bike access on private land. The trail design can be more "creative" and trail networks on private land are generally not subjected to the same level of scrutiny as trails on public land.
One of the biggest obstacles to gaining access to private land are the land owner's concerns of exposure to liability. There are a number of tools that we can use to engage private land holders to allow mountain bike access.
The following page at IMBA has a great discussion on the issues of liability for land owners.
The relevant section for private land owners concerns "Recreational Use Statutes". There are a few important points to note:
"Recreational Use Statute" is a term given to legislation generally intended to promote public recreational use of privately owned land. The statute does this by granting landowners broad immunity from liability for personal injuries or property damage suffered by land users pursuing recreational activities on the owner's land."
Justification for the RUS policy:
"The underlying policy of a Recreational Use Statute is that the publics need for recreational land has outpaced the ability of local, state, and federal governments to provide such areas and that owners of large acreages of land should be encouraged to help meet this need."
How it works:
"Under prior common law (law made by precedent), the landowner had different duties of care depending on whether a person was on the land as an invitee, licensee, or a trespasser. The greatest duty of care was owed to an invitee and no duty was owed to an unknown, adult trespasser. Under an R.U.S., recreational users are treated in the same manner as trespassers and thus the landowner owes them no duty of care. The protection of the statute is lost, however, if the landowner charges for the use of the land or if the landowner is guilty of malicious conduct."
Generally, a land owner is not liable for recreational use on the land if they do not charge for access and there is no malicious conduct on the part of the land owner to create a dangerous situation.
How this may manifest itself as challenging trails on private land:
"If a landowner has no knowledge of a dangerous condition, he or she is under no duty to investigate the land for dangerous conditions."
The full text of the California Recreational Use Statute is here. A summary brochure can be found here.