Here's an article I found:
In Los Angeles (Southern california) the Mafia Mexicana predominates
and it adopted the number of its first letter: 13 (M is the 13th
letter). According to that, in the gang's culture, the number 13 is
used to indicate the Los Angeles area. Like the MARA SALVATRUCHA is a
Los Angeles-based gang they adopted the number 13 for them, this is
why they use as initials MS-13; MS for Mara Salvatrucha and 13 for Los
Angeles.
San Francisco (Northern California) is the territory of Nuestra
Familia and they adopted the number 14 (N is the 14th letter), this is
why the Mara's members of this area adopted the initials MS-14.
Note: the Mafia Mexicana and Nuestra Familia are criminal
organizations that control the activities of the latin gangs from
jails in their respectives territories.