This is a very good question. I had to think about this awhile.
The word "some" is usually an article like "a", "an", or "the". In English, we never put an adjective before an article. If there is an adjective like "new", it should be between the article and the noun. We say, for example, "the new thing" but never say "new the thing". So... even though "some" is combined with "thing" to form its own word which is a noun that should be able to have an adjective before, we never do that. You can say "some new things" but you can't say "new some things".