Because NPAPI was basically a backdoor in the browser which has been abused by malware for a decade, at least. It wasn't safe and it couldn't be upgraded easily.
Also this standard refers to add-ons/extensions, and not "plugins". Browser plugins are dead, and have been for a while.
Also this standard refers to add-ons/extensions, and not "plugins". Browser plugins are dead, and have been for a while.