L.A. has actually been much better than NYC at building new transit infrastructure over the last 40 years: https://archive.ph/sbTkC. NYC, though, is obviously starting from a much higher base. L.A. should also be working harder to build more faster, but it's not the laughing stock it once was.
You're very generous with LA. The few lines it's built recently (eg Exhibition Line) are so slow that no one in their right mind would take it unless they have absolutely no better option. In Paris, public transport is always the first option.
The speed yes it’s not ideal and at a minimum there should be signal priority or just remove at grade crossings entirely, but land use in LA is really really not good for transit. The built environment is designed to support automobile use and that will hopefully change through TOD.
My point is the distances regularly covered in LA would be absolutely unthinkable in Manhattan and the more urban parts of the outer boroughs, let alone Paris which is a smaller city than San Francisco. So on the one hand we need better land use to reduce distance traveled and on the other improvements to overall speed.
By contrast New York has a built environment that is absolutely fantastic for transit but cannot build enough to fully capitalize on it. Their costs are astronomical and going up.
Traffic reached a tipping point where rail was a viable if not better option. The pandemic changed the calculus a bit but in general never underestimate LA traffic.
Thirty years ago, the rule of thumb was that to get from point A to point B for all points A and B in the L.A. area it will take about an hour. I think these days that rule of thumb has gone up to 90+ minutes, although it’s over a decade since I left L.A.
One thing people don’t realize is that the LA Metro area actually has a greater population density than the New York Metro area. Yes, Manhattan and some of the boroughs have higher density than you’ll see anywhere in Los Angeles, but the low-density areas in L.A. are higher density than the low density areas in NY.
If you start digging into statistics, LA hasn't been getting all that much value out of its public transit investment. Using 2009 and 2019 as base years (partially because 2020+ are screwed up because of the pandemic and partially because that's where I can actually reliably source data quickly), the LA metro area went from 6.2% mode share in 2009 to 4.1% in 2019. NYC increased from 30.5% to 31.6% in the same time frame, and most of the other US cities-with-decent-transit posted similar small inclines (DC being the big exception, but this decade saw an epic meltdown with WMATA, so that's to be expected).
