Because, as stated in the signature campaign rules, you have to write #JOIN, not #Proof or any other unvalid tag.
If you want to be accepted and added to the signature campaign spreadsheet, you have to make the change and quote this message again.
Good evening. You have written that members can participate in the subscription campaign, but for them there are no signature options. So full members are allowed to participate?
"Participants in this campaign have to be at least a Full Member to wear our signature." from signature campaign rules.
As stated in this blog post, I am posting this on behalf of Sylon.
I am personally not involded in any kind of bounty campaign management, so don't send me any PM, because I will not answer.