It's what they were using last season (e: https://www.vlr.gg/70994/mandatory-vs-team-vitality-valorant-regional-league-france-revolution-week-3)
Every single team in the world can lose a map against anyone, if you want to have a league that has the best teams to come out on top, then you want to have their performances not reduced to a single map. Of course bo2 isn't ideal, but it allow them to not lose an entire week of preparation because they fucked up one single map. I don't know if you would call that bo2 or just two maps but the idea is the same