If Manchester doesn't have daily LAX or SFO yet, how on earth do the airport's aspirations extend to San Diego? It's a much smaller market than either of the others.
SAN is an attractive leisure destination and cruise departure point in its own right, and probably the airport is targetting no more than a once or twice weekly service from the likes of TCX, TOM or VS.
These days people want point to point and aircraft like the 787 or A330 make it feasible. There's no rule book which says you have to have daily flights to a traditional mega hub before you can launch a direct flight to a smaller attractive destination in the same country. If that were the case MAN would not yet have direct flights to the likes of Goa, Phuket or (soon) Sanya.