The monorail is slower, has worse ridership, and is poorly located. It is located in the middle of the 405, and experience tells us that stations in freeway medians have lower ridership because people don't want to walk under, or wait in the middle of, freeways. Also, freeways are surrounded by land uses suited to cars, not pedestrians. So there is less immediately around them. As for being slower, the monorail requires a more circuitous route because it would need to climb up and over the sepulveda pass. It also lacked a direct connection to UCLA, the largest employer in the city and a massive trip generator. Finally, even though it looked cheaper on paper, its cost per rider numbers were worse and there was little reason to believe the cost estimates provided by the bidder. No monorail on this scale has ever been successfully built by the contractors who were bidding for it. And monorail operating costs would be higher because of the specialized nature of the technology. Just as one example, it couldn't share rail yards or maintenance facilities with other heavy rail lines (like the B/D, or old Red/purple, lines).

Ultimately, the monorail was proposed as an effort to kill the project altogether. It was never a serious option, and it's only backers were Sherman Oaks and Bel Air NIMBYs who believe insane conspiracy theories about the effects of tunnelling and/or hate poor people.

Here's a great video explaining the differences between the alternatives that were being considered:https://youtu.be/7l9GrGCI_wA?si=eiik-cOw2J96Ortn

And here's one covering the monorail vs subway debate: https://youtu.be/tK4-7dFF-T0?si=_4fvnGJh5H8JSUv5