The best reason I've heard, is around redundancy/robustness. HTML recovers from invalid syntax and missing resources, it'll still function as a page displaying

The best reason I've heard, is around redundancy/robustness. HTML recovers from invalid syntax and missing resources, it'll still function as a page displaying content with functioning links to other pages and such. Same can't be said of JS, one missing resource or syntax issue and it can break, which depending on how much you rely on it can leave the page unusable and a user stuck.