Campaign manager usually choose member based on their merit count or else it shouldn’t be stated during the application or on the campaign thread itself.
Is this confirmed by a campaign manager, or is it your assumption? If someone can't earn 5 merits in the last 120 days, they are likely a spammer, while having earned more than 120 in the last 120 days doesn't automatically make him quality poster. I'm sure every other manager who has this requirement in their campaign thread must go through other details before accepting a user. Though I can't speak for others (everyone has their own way, and if you aren't lazy enough, you will find their opinion on this matter from long ago), I myself value merit sender + posts where merit is earned more than the total number of merits earned. For example, I would gladly accept a user who received 5 merits in the last 120 days in the technical discussion board, while rejecting a user with 120 merits earned in a local from some unknown user ( no discrimination, not being racist at all lol, that's a more easy way to identify a genuine quality user).
Well, the number of campaigns is way too high these days, and at some point, everyone gets accepted lol. You have to fill the slots, you can't keep the weekly budget unspent for months.