Because by her own admission, Sigourney Weaver said Ripley was always written a certain way and she felt David Giler and Walter Hill "Got" Ripley, whereas everyone else aside from James Cameron wrote her like a Basketball coach. The last time we got "A new take" on Ripley, we got Ripley 8.
Nah, Ripley's had a full arc and then some on the side, time for a completely new character with no resemblance to her to take protagonist centre stage. Not an antagonist-protagonist (David) or a Ripley mixtape.