Red Superfly
You creepy little stalker
Well, it comes down to licensing.
Soul Caliber appears on a Nintendo machine, so Nintendo allow one of their characters to be used.
Soul Caliber has Todd McFarlane on the design team. He came up with Necrid. McFarlane gives the OK to use his Spawn character.
Soul Caliber appears on PS2, which also is home to Namcos OTHER beatemup, Tekken, so use Heihachi.
Do you seriously think Squaresoft would allow for Namco to put in a character of theirs into a game? C'mon. People have suggested Dante from Devil May Cry because he has appeared in Viewtiful Joe - NEWSFLASH - Dante is a Capcom character, as is Viewtiful Joe, Capcom can do what they like with it, Namco can't.
The only realistic alternative would have been to use another Tekken character instead. They were already using Yoshimitsu, the best possible candidate for a Soul C game. Heihachi was a good choice.