All four of the major "digital assistant" programs--Siri, Cortana, Alexa, and Google Now--have female voices. Google Now used to have a male British voice, but a while back, it switched to an American female, with no option to return. They want you to think of the programs as a person--and that hypothetical person is female.
Because when you're giving orders to an automaton that has no choice but to follow them to the best of its ability, it should code as female, right? Wouldn't want people thinking a baritone-voice Brooklyn accent would carry out your commands all day. Wouldn't want people giving commands to what sounds like a man who works as a California newscaster--myghods, they might get into the habit of giving orders to anyone who sounds like that!