Las Vegas/Venturas has casino heists, two relevant streets, and some drunk people. Organized crime is virtually nonexistent, and the mob is dead in Vegas, before someone tries to bring those irrelevant has-beens up. Most of the Las Venturas missions in San Andreas involved the Caligula's Palace heist, or had you driving out to the desert for something.
San Francisco is one of the most diverse, cultured, and interesting cities in America. It has more landmarks than any other GTA city, aside from NY/LC, it's significantly larger and more dense than Las Vegas/Venturas, it's home to Triads, NorCal Mexican gangs with cartel ties, and a much more relevant criminal presence than Las Vegas, overall. The city is more scenic and the hills are more fun to drive through than the aforementioned two relevant streets in LV, and SF is home to a significant street racing scene.
I can keep going, if you want.