A few reasons:
#1 is the refusal to optimize the roster based on a desire to play Wemby at the 4, try Sochan at PG, and keep Tre Jones on the bench. The obvious starting lineup was:
PG Tre Jones
SG Devon Vassell
SF Keldon Johnson
PF Jeremy Sochan
C Victor Wembanyama
Five players playing their natural position (yes, 7-4 Wemby is a C). That lineup has a +41.3 net rating but they've only played 21 minutes together, the best of any lineup with 10+ minutes.
Ability to view statistics and advanced analytics for any combination of 5-man, 4-man, 3-man, or 2-man lineups that a team has played throughout the season. Lineup information dates back to the 2007-08 season.
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#2 is related. While Wemby is a fine ball handler and creator for his size, he really needs a true PG to get him the ball in his unique spots. But this team is mostly bunch of scorers who generally create for themselves. Wemby is a cheat code only a few of teammates can figure out how to work.
#3 is talent. I fear these guys may not be as good as I thought they'd be. I figured this team would be good for at least 30-35 wins and just needed a veteran PG and star wing to make them a legit contender. Wemby may be the only one who is a legit top 3 option for a contender. Vassell may be a limited streaky shooter and not much of a playmaker. Keldon Johnson seems like a more all-around player. Collins is a good complementary big. He's a good heavy to pair with Wemby and he's a good interior passer. Pop is trying to turn him into the stretch 5 he was at Gonzaga, but it's not really happening. It's hard to judge Jones without giving him full-time PG minutes. It's hard to judge Sochan as a PG rather than a high ceiling big wing. The bench is decent.