1st reason being that my current charger, Onyx 235, and many others only support up to 4s. 2nd reason previously stated. If you get a bad cell in a 6s pack you have to replace the whole thing, if running 2x 3s packs, you theoretically only need 1 new 3s pack (half the cost). I've found the cost to be similar for a pair of 3s packs or 1 solid 6s pack.