From the simplest explanation Lord Victor gave him, it seems that after the Star Monarch Era, the heavens had made it impossible for Supremes to become Celestials.
In fact, in the past billion years, the number of Celestials that appeared in this known universe is less than a dozen, and they all came from the Heaven's Court.
Only ten Supremes were allowed to become Celestials. The rest achieved that milestone in the Forbidden Universe after spending billions of years there.
However, they returned as heaven's rejects, being rejected by heaven. However, since they came back as Celestials, they didn't care too much about the ill treatment the heavens were giving them.
Regardless of what the heavens thinks or how cruel it acts, they had become Celestials, and that was what mattered to them.
It was like someone felt threatened by Celestials, and so they had made it impossible to become a Celestial in this universe.
