Because, when someone is a God , he is THE CREATOR and THE RULER of EVERYTHING in the EXISTANCE! For example, there cant be a ''Water God'' or ''God of Seas'' because that God cant controll andything that is out of water (things like fire and air, for an example) so he is not almighty, and thats the reason he cant be God, and thats the reason all the pagan Gods are false. There can ONLY BE 1 GOD,
That's only by your very narrow definition of god. Not anything official. In any case there are multiple other religions that have a supreme creator here are a couple of examples;
http://en.wikipedia.org/wiki/Sikhism
http://en.wikipedia.org/wiki/Zoroastrianism
http://en.wikipedia.org/wiki/Tenrikyo
http://en.wikipedia.org/wiki/Cao_Dai