When I was trying to figure out how the band could comeback there was not a single scenario in which I imagine having a female vocalist as the lead singer, I love female bands its what i listen mostly but I think that female vocals dont go well with LP's sound.
But no matter what I'll support the band, hopefuly they chose the right person for the job.. being the lead singer of LP means that you MUST be able to sing almost any genre, any style.
Why a girl? the only thing that comes to mind is that they want a fresh start, probably having a girl would help to not being compared day and night to Chester.