Reason why not voting is bad- We won't know who is mafia with 100% certainty unless the person who is mafia straight up claims to be it. So no matter which day, we have to rely on our deduction.
The problem with not voting is that even if we don't kill anyone Day 1, if we kill on Day 2, the game still ends in the same amount of time and we get one less chance to choose who we think is suspicious.
Just based on percentages alone, if we don't vote D1 and vote D2, there will be 4 players on D2 so 25% chance of catching the mafia.
But if we vote both D1 and D2, 20% chance on D1 and 33% chance on D2, which is a lot better. (Overall that adds up to a 46.66% total chance of catching mafia, if I didn't botch the math)