Surenos "hold it down" in the Bay Area because they are actually present in the Bay Area. You don't see nortenos holding it down in southern California because there's very few of them down there.
The reason is fairly simple. Field work brings southerners up north. Nothing really lures northerners down south. If the situations were reversed and most of the agricultural work was in southern California, you'd likely see a reversal in the presence of the gangs in both regions. In other words, you'd see nortenos in the south, but few surenos in the north.
Obviously there isn't much agricultural work in the SF metro area, but the existence of such work is the reason they are prevalent in the region.
The reason is fairly simple. Field work brings southerners up north. Nothing really lures northerners down south. If the situations were reversed and most of the agricultural work was in southern California, you'd likely see a reversal in the presence of the gangs in both regions. In other words, you'd see nortenos in the south, but few surenos in the north.
Obviously there isn't much agricultural work in the SF metro area, but the existence of such work is the reason they are prevalent in the region.