Minerals and royalties can become cumbersome to split in an estate. Selling allows you to simply leave cash to your heirs.
Managing mineral rights and royalties can be more trouble than they are worth. Their amount is unpredictable and often small once interests have split from older generations, they generate a lot of mail, and most people find they need to find a specialized CPA to do their taxes. By selling for a lump-sum payment, you can invest your money into something more predictable and easier to manage.
The cash flow from mineral rights is irregular and, even for people in the industry, largely unpredictable. Additionally, sometimes people could use a lump sum of money today rather than waiting many years to get the same amount out of your minerals that we typically offer.