The sequence must have been: first the clearing with ash trees growing in it; then the surname, for the owner of that clearing or the bloke who lived next to it; then from about the seventeenth century onwards women started giving their maiden names to their sons if they didn't have brothers or they came from a more distinguished family than their husbands! The idea was to keep the surname alive, not to create a new first name. And more sons get named after their fathers than girls get named after their mothers, so this was a safer bet.
However,
Ashley has got the same sort of soft, flowing sound that has made
Cathy,
Sally,
Hayley,
Natalie etc into standard-issue, one-size-fits-all female names, and fashion supports this because boys are currently undervalued - I think; perceived as potential petty criminals and dropouts who will grow up to be serial rapists or MCPs at the very best! But, ironically, giving a girl a (previously) boy name seems to magically make her strong, noble and independent; all the good things that boys used to be!
Not that I can prove any of this, you understand! But it makes sense to me from what I see happening. I'd love you to comment, if you agree or not - it is interesting.