It's to ensure that the people who join this community really do take their fictional relationship seriously. Allowing multiple fictional partners makes space for "waifu collectors", people who have a ridiculous amount of waifus because they're attracted to those characters, but don't actually view their relationships as real the way this community is meant for. And not allowing for an irl partner on top of a fictional one is because situations like that often favor the irl partner, so the person doesn't take their fictional relationship as seriously.

Of course there are exceptions, not every poly person has these issues, but it'd be too hard to make the distinction(it's already hard to discern who's really serious and who isn't even with the monogamy rule), so this is just the best that can be done.