A lot of people have asked "why do Bomberman's bombs explode on a grid?" The answer is simple: See the hard blocks are indestructable, so the flames can't go through them, they have to go around them. Since the blocks are all aligned on a grid pattern, the flames only have that path to follow, since it is in the nature of fire to follow the path of least resistance, in this case, the grid.
So there, now that's been cleared up. Now, it would be more realistic if the powerful bombs also flooded in different directions, but seriously, that'd just make the game too hard and not much fun, plus it's a bitch to program. Believe me, I've tried.