I have a lot of respect for you BUT didn't Harris compete in 2020, how did that work out?

She wasn't Veep then. Being Veep is an enormously advantageous position to be in. Every elected Democrat in the country will take your call. Every donor, every fundraiser, every person of the slightest importance in the party will meet with you. You have a national platform that no other contender can match. You have the ear of the President, and possibly his endorsement (explicit or implicit). Etc.

Plus, the issue set in 2024 matches Harris' background better than 2020. That wasn't the year a former prosecutor was going to win the Democratic party nod, and Harris had no way to work around that. This election cycle (and especially in 2022-2023), the national mood had shifted away from "criminal justice reform" and back towards "law and order" - with Democrats fleeing from anything that sounded like Defund the Police.

To say nothing of the fact that a lot of eventual nominees had an unsuccessful run first. A lot of Presidential nominees lost their first race(s): Reagan, Bush, Hillary Clinton, McCain, Romney, Biden, etc. It's more common than someone winning the nomination on their first effort.

So yeah, in this counterfactual world where Biden declares he's not running again, Harris starts the race for the nomination as the favorite.