I'm oversimplifying but...
Surf was a founding member of ECNL but jumped ship to DA when they thought it was a better play. When Surf + other teams started moving to DA ECNL brought in different teams to take their place. This is how Sharks were brought into ECNL.
Last year DA blew up and Surf, Blues, others went back to ECNL.
General FYI, A couple of years ago Surf tried to "buy" DMCV Sharks like they recently "bought" SDSC. I'm using "quotes" around buy because it wasn't a true buyout. More like a patching over but they would never be part of the Surf SD club. Sharks leadership put joining out to a vote and Parents chose to decline Surfs offer.
Culturally Surf and Sharks are very different. Surf has no problem cutting / moving players around. Sharks want to win but they're a little more sensitive to the repercussions of cuts.
For what it's worth there's a lot of SD parents starting to reach out to Sharks because of ECNL now that DA's gone. Over time assuming ECNL does't offer membership to any other SD clubs the Sharks record will improve.