The reasons why range from "people are reminded of their mother and this is relaxing" to "People are still so freaked out by HAL in 2001: A Space Odyssey that they would be terrified of male electronic devices talking to them."
Given how many people interact with their mothers and the fact that most consumers under 30 have probably never seen that movie, I find both theories doubtful at best.
But, I'm not here to talk about that. I'm hear to say what voice I would want if I ever owned a car and that car happened to have a GPS system.
I'd want Samuel L. Jackson.
"Make a left, motherfucker at the mother fucking next light."
"I told you to make a motherfucking left turn motherfucker! I'm tired of you not making mothefucking left turns when I mothefucking tell you to make a left turn."