Because the maximum crit rate you can get from rings is 30% so Merg + RoL1 + RoCham4 is enough, and you also have some little magic from Ring of Champion. Compare that to Merg and 2x RoL1: you waste 5% crit rate and no "little bonus" from Ring of Champ4.

Best accessories (imo): Merg, RoL2, Leth, RoS10, AoP6, 2xBoS7, 2xVE7.