Many good reasons to lack a red lightsaber crystal, or purple for that matter.
1. 4,000 years before the Battle of Yavin--Exar Kun's time--There were no synthesized crystals that projected a red lightsaber blade of hatred like, say, 1,000 years B.B.Y. or even after then. Even though KotOR goes against this established part.
2. As there were no other kinds of crystals except for blue and green around him, he might have taken a less Jedi-like color. It's just like how if you find a lightsaber from a dead Jedi, you wouldn't care what blade it projects would you? It's a lightsaber and it's pretty effective. That's why Exar Kun took some crystals that projected a blue blade. And if he had gotten it from Illum, that's another good reason, as it is rich in crystals that project lightsaber blades.
And did you know, before the Sith started using synthesized crystals, they used natural crystals that projected a purple blade? That was normal I believe, especially at times such as the battle of Ruusan, which was 1,000 years B.B.Y. and at the time which the aftermath created the Valley of the Jedi (creating a storyline for Star Wars: Dark Forces 2 - Jedi Knight).
Oh, and the true symbolic meanings of lightsaber colors
. I didn't think I'd have to post this URL again (at a more specific direction).