“....why he became Mayor.”
Stepping stone to becoming governor. Political power for the sake of holding political power. Not to make life better for the peons, but for sheer self-aggrandizement.
Make promises you cannot or will not keep, project your failures upon opponents, and make sure your patrons are all on board as you jump to do their bidding.
Garcetti is still in training -- He's Newsom's (aka, Nancy Pelosi's nephew's) "apprentice".
Before he is "allowed" by California's extreme-Left Democrat Party Garcetti must prove he has the ability to cause unconsolable financial pain, massive demoralization and depression and even death at a much larger scale than just to the citizens of L.A. To do what he has "accomplished" to California's 40+ million citizens requires practice and proven results ... which Garcetti is working hard to produce.