Unless you can be concrete and specific about some of those reasons, you're just replacing handwaving with more vigorous handwaving.

What is it specifically about JavaScript's implementation of data isolation that, in your mind, helps cause the excessive memory usage?

Just a day or two ago, there was an article here about problems implementing a kind of read-only memory constraint that Javacript benefited from in other OSes.

I must have missed that article. Can you find it?

Unless you can come up with a specific reference, it seems unlikely that this would explain the large memory efficiency difference. By contrast it is simple and straightforward to understand why keeping temporary garbage until garbage collection could result in tying up a lot of memory while continually running code that allocates memory and lets it go out of scope. If you search, you'll find lots of references to this happening in a variety of languages.