Because lord Shiv has given Ganesha a blessing that each worship would be accepted if it is started by ganesh puja thats why ganesh pooja is the initial worship of god.
I will tell you a story.
Once all gods an godesses were in trouble about Who wwould be worshiped first. then Narad suggested that he who could come quickly after a visit the universe , would be worshiped first.
All were come out with own vehicle except Ganesha.Narad asked Ganesha the reason of not going. Little Ganesha replied that Lord Shiva and Mata Parbati were his universe,so he moved around his parents.
Hearing this LOrd Shiba pleased and blessed him,"you will be worshiped first among all gods and godesses".
Ram Prosad, Even I heard about this story when I was a young one but never knew that it was related with this!!!
Very well explained on your part and I really enjoyed going through it once again and it rejuvenated my memory once again... :)