how do the organizer or manager of a campaign chose the participants?
Base on the campaign manager's preferences but likely to fall only on one category; not a shitposter, or not just making low-value contents or one-liner posts if you are only talking about a signature campaign.
i noticed that some aren't good at posting and still earning btc.
Those are handled by an incompetent campaign manager, to be honest, they are not earning directly with bitcoin rather on the company/projects tokens. If you examine participants paid with bitcoin most of them are handled by good managers and more likely strict to the quality of contents (posts) of his/her participants.
I concluded that ranks doesn't define your ideas because when it comes to campaign it is just define the amount of your payment and still lacks ideas on posting. any thoughts?
Exactly! You may have five shiny coins below your name but your idea is for a newbie or just want to reach the number of posts over the week because you got a paid signature wears on your profile, that doesn't really define it.