@owlette In the authors note at the end, they mentioned that they think of the main couple as a "riba" or "reversible" couple. More generally, the reason BL tends to stick with strict seme/uke relationships is, well, outdated understandings of gay relationships and the traditions of the publishers and fujoshi communities.
While "reversible" or "switch" couples are becoming more common in BL, there's still a fundamental misunderstanding in the community that someone in a gay relationship has to be the "man" and someone else has to be the "woman." It's basically a stubborn stereotype that—as LGBTQ life and issues become a part of mainstream Japanese discourse—has begun very slowly changing.
Another reason it sticks around is that many editors and publishers believe that a story must have clear delineation between seme/tachi/top and uke/neko/bottom in order to be marketable. That belief isn't helped by the enormous section of the fujoshi community that enjoys discussing who in any given ship would be the seme and who would be the uke. For whatever reason in that community, suggesting that characters would switch sexual roles (or not engage in penetrative sex) is considered radical.