The exact same reason an individual or corporation sells the real property or real estate. No difference in why royalties are sold and liquidated. To raise cash! The most valuable properties are sold on a regular daily basis and include royalty properties. Often times an oil company will liquidate part of its inventory of oil and gas royalties. This is to finance a particular drilling project or for corporate overhead expenses.
An individual oftentimes is confronted with cash needs which may include, an estate liquidation, tax issues, lawsuits, community property issues, or raising cash to buy another property. Valuable oil and gas royalties are sold every day, for common issues requiring cash needs.