Well the main storyline had been completed already after all. All that was left was improving how the game feels and play overall: dungeons, battle, etc...
But when he started, he had little programming experience. Now that he does, there's plenty of stuff he'd like to revamp. So much so that might as well make it a new game. It also helps from a financial viewpoint: fewer people will pay for improving an existing game. Creating a brand new one with a new storyline, improved elements etc though? Now we're talking!
It's a bit of everything, it all adds up and led him to this natural conclusion.