Even if the X-Men were being promoted better, the premise of the franchise has remained stuck in the past for a while and wouldn't really be marketable these days anyway. This idea of using mutants as an allegory for civil rights might have been novel two decades ago, but given how society has changed since then and how our understanding of social justice has evolved and adapted, you can't really a story about how shooting laser beams from your eyes is the same as being gay, invariably written by a straight dude, and expect that to click with audiences in 2021.