This is most likely because your profile does not meet the campaign’s criteria. The reason for this can vary from reach, your location, engagement, or gender depending on the campaign. In each campaign there is a section with the 'Creator Requirements'.
To understand our briefings better, we created a blog post.
If you have any further questions about your eligibility for a particular campaign, please email us at firstname.lastname@example.org or chat with us via the orange bubble in the bottom right corner.