I think we can blame smartphone operating system developers for this. For so long they dug their heals in on only allowing certain languages run on their systems. But they also put a web browser on the phone, and then worked to make that browser match the desktop version as closely as possible. It was an escape hatch to be able to write cross-platform code, once and for all. Oh, sure, Android ran "Java", but it didn't include enough library commonalities with desktop to work. Same with iOS and Objective-C (which is practically unheard of outside of the Apple sphere). I've had far easier times getting one binary to run in Windows Mobile, desktop Windows, and Linux.

So I think there are very good social reasons why JS "won out." Perhaps it was the weakness of the language itself that made it possible. It didn't seem like a threat in 2008.