People in San Diego used to treat Tijuana like part of the city, just split by a border is all.

And I have family that still visits there, but it is much more dangerous than it used to be. 30 years ago it was basically "don't drive a brand new vehicle and be polite" and now it's all sorts of things including knowing which highways to avoid, how to stay away from parts of town that may have drug lord activity, etc. There was almost an unwritten rule years ago that you don't mess with normal people or tourists; that rule has been significantly broken.

And the border waits are much more annoying, too; the people who do still cross now have to wait much longer, even with Sentri.

drug war, notably shift in power from Colombia to Mexico. Various power plays between different organizations advancing on eachother and government interventions. Much more dangerous for civilians in the 'wrong place at the wrong time'. But violence against tourists is by and large greatly exaggerated, even at the height of narco-violence in Mexico. It is still a safe place for a tourist in every single city and most rural areas.