I actually started with Left,Down,Right since that made sense to me. However having played it a bit those keys felt too close together, only really allowing to play it with one hand.
Spreading them out like this allows you to play with both hands, and roughly as it is on a real drum set.
I guess it depends on each person's coordination and fingers size, some completely fine with doing all three with one hand on the arrows and some preferring to spread out. On a real game a keyboard mapping would solve this :D
Come to think of it, I might just add multiple mapping. No reason not to!