Because inclusion is trivial. Everyone is welcome. The process of exclusion is what causes you to define the borders of acceptability, and allows participants

Because inclusion is trivial. Everyone is welcome. The process of exclusion is what causes you to define the borders of acceptability, and allows participants to have a clear understanding of when they're at risk of being excluded. Then they'll weigh their wants vs the rules and make a decision about whether they value their ideas over the group membership. Then what you are left with is people who want to be in the group more than they want to follow their whims against the group's rules.