Because the scores were level after 14.3 overs bowled, and a 4 or 6 in the 4th ball of the over leads the team score to be calculated from 193/5 or 195/5 as was the case, thus giving MI a better run rate. I know the commentators were going on about how a wide in the 4th ball would've sent RR through to the playoffs, but that's blatantly false as a wide would leave it at 14.3 overs bowled and MI qualifying with 190/5.