Number of syllables. "Angeles" has 3, and is already too much. So they say L.A. (2 syllables). It's the same reason you call New York in full (not N.Y.), as i

Number of syllables. "Angeles" has 3, and is already too much. So they say L.A. (2 syllables). It's the same reason you call New York in full (not N.Y.), as it's already 2 syllables.