As others say, it's about the size of the port. But it's not only that those ports can't handle more ships - it's also that many of the ships in operation are so big they can't fit in smaller ports. I know that Oakland, for example, is always going to be second tier because it's just not possible to send the largest container ships into the San Francisco Bay without rebuilding a lot of the port to handle much bigger ships - dredging, larger cranes, larger docks.

This phenomenon is because larger ships are more efficient, and ocean shipping is a super competitive, low-margin business (during normal times) like the airline industry, so in order to eek out a tiny profit, ocean lines consolidate and build bigger and bigger ships, which further drops prices. These ships can only actually go to a handful of megaports around the world, though.

Another factor is the infrastructure that exists at the ports - you need to do something with the cargo once it gets on shore. LA/Long Beach has invested in big, efficient terminals for trucks and trains to get loaded quickly and move cargo out of the port in big volumes.

Source: I used to work at Flexport on ocean shipping, so I have a general understanding of the industry