Female dwarves were on the wishlist/roadmap for a long while. We had some internal concepts, but it never got to a proper place.
It's not really about the voicelines, although that is a giant workload too - in 2022 we had 3669 voicelines for the dwarves, so all of those would have to have a female equivalent.
The biggest issue is the character rig, that's ancient and have only been made to accomodate the single potato-ish body type. We'd have to redo that from the bottom up, remake all the cosmetics to fit the new body type and so on.
TL;DR: Redoing the systems to add the female dwarves we feel the game deserve is such an enormous task that it's not currently on the table.