Because it isn't just about points.
Rielly isn't good enough defensively. Straight up. On top of that, his offensive numbers are greatly boosted by an unsustainable start. As an example, 9 of those 20 goals that are being brought up were scored in the first 20 games. After that, his numbers started to drop. I haven't actually calculated those numbers after his hot start, but I suspect they fall under the "quite good, but not Norris-worthy" and it would be ridiculous to say he's Norris-worthy on the defensive side. When it really comes down to it, I think Rielly started very strong, but it was clear he wasn't able to maintain the offense that drew so much attention early in the season and his defensive game just wasn't good enough to make up for that.
As someone else said: He would have been jobbed if the voting had taken place early in the season. It wasn't, and the Norris isn't awarded for 1/4 of the regular season. As it is, he'll still finish strong in voting, but I don't think he was one of the three best defensemen in the game from the start of the season until the end. He was only in that discussion for the early part of the season.