Well it still exists and is actually great for distributing Java desktop apps inside a corporate intranet.

But it never caught on because it was a poor implementation of a dubious idea. It started up many hundreds of time slower that Flash, required you to context shift out of the browser, was pretty ugly, and obfuscated the fact it was getting unrestricted access to your machine behind confusing security dialogs.

I think the underlying programming language was the least of its concerns.