In terms of eSports in general. Not just Valorant
bo2 have the advantage of always being exactly 2 maps long (so great for the organizer who can then make a set schedule and sell a guaranteed amount of sponsor spots), with the disadvantage of draws being possible
bo3s have flexible map count (2 or 3) but there are no draws
so bo4 would have both disadvantages (variable map count 3 or 4, and the possibility to draw)