This is just my own personal hot air spewing, and may or may not be worth the bytes you're reading.
TIJ isn't much of a "destination" vacation draw.
TIJ isn't much of a business market.
TIJ isn't much of a family visit market (at least not from air distances).
You'll note that CJS, another huge border city has the same issue.
Also, note that your typical Mexico international airline ticket has over $100 of various taxes and fees. While a SFO/SAN flight might be $175 RT, the same ticket with the airline making the same amount of $ would be about $300 for SFO/TIJ.