Why don't we use the Septuagint numbering for the Psalms these days?

It's a good question. So when I have to quote the Psalms, I have to give both the Septuagint reference and the Hebrew reference. Why that happened: it's the same reason why we don't say Paralipomenon for Chronicles. Paralipomenon was the Septuagint's title for those books. Chronicles is from the Hebrew, not transliterated but translated. So that's why we have First Kings, Second Kings, Third Kings, and Fourth Kings in the Catholic Bible, whereas in the Hebrew version we have First Samuel, Second Samuel, First Kings, Second Kings. It just happened. There was a great transition, and that was from the third century BC to the first century BC, because the Septuagint was from Alexandria. There were more Jews that were Greek, especially in the Alexandria area, than there were in the Palestine area. And so there was a transition going on between Hebrew and Greek, because Greek was the lingua franca. It was the language that everybody spoke. Everybody knew Greek. I'm sure even the Pharisees knew Greek. You had to if you were going to get along in business or family life, because you may have had relatives from Alexandria. Who knows? So in that period, the intertestamental period it's sometimes called, you had this transition going on from Hebrew to Greek. Not that the Hebrew totally died out, but for the New Testament, the Greek was quoted more than the Hebrew. Not in every case, of course. And sometimes Paul would choose whether he wanted to quote from the Greek or the Hebrew, and sometimes he had a theological reason in mind, not just a preference of liking Greek better than Hebrew. He was a Hebrew. So if there's any allegiance he might have had, it would have been to the Hebrew. That was the language of his fathers. Well, we find the Greek quoted more than the Hebrew in the New Testament. And that's okay. As long as the translation is accurate, it's accepted. And we could say that the Holy Spirit inspired the translation to be put in the New Testament. We can go that far. So it's just one of those things.