Because different sites are different communities and have different norms and standards. Each site's meta site is supposed to be for discussions about that particular site or community, by active members of that community.
ServerFault and MathOverflow are good examples of communities where the standards and norms are radically different from the rest of the Stack Exchange network, in that they are explicitly for professionals, rather than anyone with an interest in the subject matter, so there is also the issue that experience and knowledge on one site/community doesn't necessarily transfer.
So that's why there's a rep restriction on meta sites. As it says in the "participate in meta" privilege page:
Because we allow anonymous participation, we require a small bit of parent site reputation to prevent spam, and ensure that meta is for active, engaged members of the community.
The rep restriction is designed as a filter, or a minimum barrier to entry to keep the meta sites for "active, engaged members of the community."