Because Bungie needs the launch sales of a new title, and there is no chance in hell they will just let that slide. Destiny 2 sold a shit ton of copies, Forsaken sold not even close to that, and Shadowkeep will sell even less. Releasing Destiny 3 will simply be way more lucrative than releasing further expansions. The whole reason why we are getting this reskin fest with Shadowkeep is because most developers are working on Destiny 3 already.