My guess is that since she's technically the firstborn, once Odin dies then she might have say over what happens with it. She had the hammer before Thor was born

OOOOOOOOOOHHHHHHHHHHHHH
if HE be worthy.
BUT HELA’S A GIRL!
OOOOOOOOOOOOOOOOOOOOOOOHHHHHHHHHHHHHHHHH

But you're not a he...

Thats the joke

XD