Because Crick and Watson got the Nobel prize, while Franklin was ignored. It's her turn for a bit of recognition. Also, I'd like it to be a woman.
Franklin isn't ignored. Everybody knows about her now to the point where they'd rather put her on a bank note than the person who actually discovered the structure of DNA.
I'm not saying Franklin is not deserving of the honour. Her contribution to the work of Crick and Watson was actually only one small part of a very distinguished scientific career but, if you put the candidates in order of notability, Crick must come higher up the list.
And personally, I think this particular slot is for a scientist. I would rather a scientist be picked for their notability as a scientist, not for social justice reasons.
I have to admit that I am rethinking my choice of Alan Turing because I think I just disqualified him. He is not really known for his science but for his contribution to the British war effort.