No question that Manu plays his best basketball when he is the primary ball handler, which he is when Hill is on the court alongside him. Fact is Manu can't hold up for a whole season as it is and would without a doubt wear down a of a lot quicker if he was asked to take on those responsibilities and play like that game in and game out during the regular season.
On the other hand, Parker has the ability to win this team games virtually by himself even if Manu is having an off night or missing time due to injuries (which seem pretty likely from here on out) Something George doesn't have the ability to do because he is a far inferior player at the end of the day.
George was a fine backup PG to Tony, sure, because he really wasn't even running point when Manu was in the game alongside him but he is definitely not a starting PG in this league and turning over the reigns and making him play a position he doesn't even play would have been a disaster here. Especially with a team like the Spurs who rely so much on point guard play.