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