No entity forces startups to convert ISO's to NSO's when early employees leave the company, but they did it because it made their offers more competitive. The legal default was ISO's that expire quickly.
It's the same. If companies hear loud and clear from both their employees and from people they want to hire, they will do it if they want to compete.
Heck, any smart company might do it voluntarily to make it harder for their employees to be poached by other companies that only offer a fire-at-will employment (the legal default).
A few posts on Medium/Substack might catalyze this change.