These are the most common reasons we have heard from owners:
Accounting and Management of the asset
Mitigate the risk associated with Oil and Gas investments
Need a lump sum for paying down debt
Other investment opportunities
Capital Gains (asset sale) vs. Ordinary Income Tax (royalties & lease bonuses)
Simplify estate distribution (when mineral rights continue to be split up and passed down, they generally have less marketability as to the value of the asset)
Life Expenses!