Really? I prefer there are a few, but mostly compatible techs in that space. This is the VM that runs almost all desktop (client) software these days. Having only one implementation would IMHO be a disaster.
End users of browsers do not have this insight into their tools.
Yes, Oracle's JVM is dominant (but not on Android, which is a huge installed base). So is Chrome in browser land. Still good that Java has many, and FF exists.
[1]: https://en.wikipedia.org/wiki/List_of_Java_virtual_machines
But a lot of us still pucker up if you mention the word "IE6" and we never want to risk going down that path again. :)
There's a balance between diversity and monopoly that can be struck.