Obama was elected to the Senate in 2005 and announced his run for Pres in 2007 so he had at least two years on the job before he announced. Newsom would have to announce literally months after he's sworn in, this doesn't seem feasible for him.
Newsom has also been running for Governor since 2014, and knows that he can have a lot more clout as Governor of California rather than throwing himself in to a crowded primary that he'd lose.
Politicians generally imo run for President when they can't see a way to go much further.
