Quite a while ago I also asked this question from BH and as misfortune8 stated, only the products provided by the companies to BH are available for review.
Some products don’t need the extra advertising or promoting therefore they are not seeking the prestige of BH reviews.
I guess there isn’t anything stopping a member reviewing a particular product under another heading in Forum but you won’t get review points either. It’s good to share a good review even if the product isn’t included on the BH review list. We all love hearing about something that is good, or even something that is terrible so we don’t get caught out.