Because THRESHOLD did so well on CBS Prime, ratings-wise? It was cancelled after one season. Who knows? That may be one reason the Powers That Be decided that regular CBS was not the right home for DISCO.
The decision to put a show on a mainstream network, on cable, on a premium channel, or streaming has less to do with the quality of the show (let alone the accuracy of its science!) than with demographics and target audiences and such. When has CBS Prime ever aired a successful space-adventure series, let alone one that lasted more than one season? LOST IN SPACE back in the sixties?
CBS's (mostly older) audience likes police procedurals and such. An edgy new version of STAR TREK doesn't really fit with the rest of their programming. THRESHOLD was an experiment that didn't work.