Why it is not for Newbie? It is a pitty.
Quite simple answer, actually.
You aware that to "reach" newbie rank, someone just need to create an account and post one post, right? (zero post will be entitled as "brand new")
I'm not saying that you are one of them or accusing you having such thought, but for now just imagine you're someone with bad intention of having easy money by milking a signature campaign. You create a hundred (or even a million if you're crazy enough) account, "reach" the newbie rank, then enrolling your accountS into the signature campaign and post several easy post like "hell yeah! Bitcoin rock!" or "Is (site name) down?" or such low quality things around the forum. There, money will flow very easily into your account.
And now, imagine you're an campaign owner that allocate your money to promote your site in hope to gain visitors and earn profits. Would you like to have those people in your "promotion team"?
There is your answer.
There also another explanation, though. Like.... you know that (although not every, but certainly most) newbie are relatively didn't know the nature of this forum and has a tendency to create a mess here and there by posting something that's against the forum rule or creating spam posts? When they did that, the campaign owner will be held fully responsible for supporting-member-for-creating-spammy-posts-by-paying-them-per-post. IDK about you, but if I am the campaign owner, I would really prefer to exclude newbie with those risks that I said above (and maybe many others).
But cheer up, it's not that hard to reach Jr. and Member rank. You'll have a lot of chance by then. Plus, you'll gain some more knowledge about cryptoworld by then, giving you a lot of understandment of what is being talked about and how to response.