Because wild venison is not assured, not consistent, and not always fully traceable. That is what farmed venison can provide, and why it forms the vast majority of what you see on the supermarket shelves.
Those who provide meals to schools, the homeless, hospital patients, prisoners, etc still have a duty of care. They will not take wild venison just because it is cheaper - if it can’t match all the standards that apply to farmed venison then that is what they will continue to buy.