A Star is Born has been an Oscar contender all three times it has been made previously, including getting Best Actress/Actor nominations for Janet Gaynor and Fredric March in 1937 and then again for Judy Garland and James Mason in 1954.
It’s maybe understandable that a studio could be wary of casting somebody like Lady Gaga in such a high profile film.
While the studio may have needed convincing, it apparently didn’t take too much to convince them.
The goal was clearly to make the movie a contender again, which meant that the person in the female lead position had to be the absolute right choice.