That old works are still being "taught" can just as easily be dismissed as dogma as it can be attributed to "significance." The courses, the curricula, the prerequisites, the notes, the questions, the answers, the study guides--already prepared. Why rock that particular boat by continually introducing new material that teachers may need to be taught how to teach.
I'm not saying that a lot (or even much) of the literature being taught isn't significant. But being taught isn't the be all/end all of literary relevance/significance. Hell, longevity isn't either, for that matter. There's scads of brilliantly written, literarily significant (and socially relevant) books that will never be taught in a classroom, let alone be remembered centuries later. Literary significance is just as subjective and arbitrary as any other personal opinion held about a book.
The greats of the past still loom large in academia, in part, because they were great first. It's not as if they're still there solely because nothing has been written since that could possibly rival them. Judging the significance of present and future literature using past works' longevity as the
only measuring stick is a mistake I think.
Books/authors are significant because large groups of people
find them to be. Not because
certain groups of people find them to be.