Why do sequels use Roman numerals?

[superchunk]

Dgc1808 said:
GoW3 logo wouldn't like as nice with a 3.
People aren't complaining and there's been a lot of final fantasy and dragon quests.

Mortal Kombat pulled it of nicely as do tons of other franchises.

Also, why did you write it "GoW3" instead of "GoWIII"