No mainstream, whether literary, artistic, or political, has any need to come up with a theory to explain itself. Theory is necessary to explain things that do not go without saying, and mainstreams cannot even see that they have assumed that they go without saying.

From the inside of the political mainstream, radicals appear in need of  explanation, but the inside is that which does not need to be explained. A  generation later, the old mainstream does require explanation, and  hence theory and historical analysis. As a result, the contemporary  mainstream can be quite critical of its historical antecedents, but the  idea of turning such critiques upon itself remains unthinkable.

This can also be applied to poetry: the contemporary mainstream reads the  Modernists, not the poets who were the mainstream back then.

[Revised version of comments on this post by John Gallaher.]