Since few rabbanim know hilchos eruvin (at least as it concerns hilchos reshuyos), its much easier to rely on a psak from a great gadol such as Rav Moshe. Rav Moshe did not issue a psak for LA, so they allow it. Some rabbanim do no recommend the eruv in LA for yeshivahliet. All the excuses, such as the LA Eruv is an omed merubeh eruv, is irrelevant, Brooklyn also has mechitzos. If it works for LA it can work for Brooklyn.
“An issue unique to LA, which I don’t think is present in the Brooklyn case, is a major 4-lane road running through the middle of the eruv. I guess the local LA rabbonim determined that it isn’t traversed by 600K cars.”
If it has a din sratya then according to Rav Moshe the shishim ribo would need to traverse the road itself. Otherwise, its tally would be included in the 12 mil by 12 mil area that it runs through. It may also have its own mechitzos, but that can create other problems.