1)  It’s too big.  Gotham City is nice, compact, things are near each other.  But let’s say you’re Batman, and for the sake of this argument, you’re based somewhere in the Hollywood Hills.  (Certainly millionaire playboy Bruce Wayne could afford a pad near Jack Nicholson and Marlon Brando.)  Now you get word that the Joker is striking in Pomona.  By the time you fire up the Batmobile, head down Laurel Canyon to the 101, and blast down the freeway out to Pomona, well, forget about it, my friend.  By the time you get there, the Joker is long gone, and you just look like a Bat-chump.

2)  Batman does a lot of swinging between skyscrapers, using a portable rocket-powered anchor and cable device.  Works great on 50 story tall buildings, but not so well on endless mile after mile of mini-malls.

3)  Batman’s costume is designed to strike fear in to criminals, who are said to be a cowardly, superstitious lot.  Sure, it’s great in Gotham, but in Los Angeles, people would just assume someone is shooting a movie.

4)  In Gotham, Batman’s relationship with Commissioner Gordon is sometimes strained by Batman’s desire to work outside the law.  In Los Angeles, and in the wake of the Rampart Police Corruption scandal, the guy wouldn’t get any slack.  A “Citizen’s Committee Against Vigilante Justice” would be formed, Earl Ofari Hutchinson would want to know why Batman apprehended more minorities than non-minorities, and the whole thing would just be a huge hassle.

5)  At night in Los Angeles, it’s either crystal clear, or extremely fogged over.  Either way, it’s gonna be tough for that BatSignal to show up.  The Signal will end up just looking like a shaft of light going up in to the night sky.  It will look too much like one of those movie premiere lights that are routinely seen and ignored around here.
 

Link:

http://www.batman-superman.com/