No doubt this may have been already covered but just to refresh some facts:
Todd Mcfarlene (Creator of Spawn) helped design the 'hidden' character Necrid, exculisve to all console versions. Now whether this is related to including Link or not; perhaps Namco thought it was unfair to have 2 'hidden characters' on Gamecube but not on PS2 or XboX. While including HeiHaChi in the PS2 version was a pleasant surprise (So far it seems he's unarmed by default), what to include in the XboX version may have been a riddle McFarlene and Namco had to figure out.
Why not Spawn? A little promotion never hurt anyone
This seems logical, but I could be mistaken