Because over 1% of the LA population belongs to a street gang!!!!
From the show Numb3rs:
1350 LA street gangs
152,000 gang members
...
And according to this 2006 census (http://quickfacts.census.gov/qfd/states/06/06037.html) there are 9.948 million people in LA.
Now it's unlikely the show Numb3rs would give fictional numbers, but even if they did the numbers would have to be ballpark.
Oh nos.. you know too much!
Plot Outline:
Working for the FBI, a mathematician uses equations to help solve various crimes.
Ah, it makes sense why you watched this!