It was only able to do that because so much had been invested in "Star Trek Phase II" starting in June, 1977. Paramount was looking at launching a TV network even back then (remember UPN in the '90s?) 13 Scripts were written, a slightly redesigned Enterprise was built and all the cast minus Nimoy signed on. To replace Spock, a new Vulcan named Xon was to be introduced:
The Phase II Enterprise:
Ilia in old style uniform:
"Work on the series came to an end when the proposed Paramount Television Service folded. However, following the success of the science fiction movies Star Wars and Close Encounters of the Third Kind, the planned pilot episode entitled "In Thy Image"[3] was adapted into a theatrical production, Star Trek: The Motion Picture."--wikipedia
BTW, that’s not the Phase II Enterprise, but the final design used in the movie—that image is the movie poster art when the movie was already being filmed, from Andy Probert’s design. Nothing to do with the TV proposal, which looked very different.