Why is Huntington Park, CA full of Mexicans?

Well, all of Calif. was once part of Mexico, dontchaknow.

That is not at all a helpful comment.  When California belonged to 
Spain and then, later for about 25 years, to Mexico, up to the 1840s,
the area was in fact extremely sparsely populated, and those that did
live there were more likely to speak native languages than Spanish. 
In other words, it isn't as if Huntington Park was "full of Mexicans"
in the mid-19th century and it isn't as if more than perhaps a very
few Latinos who now live in Huntington Park can trace their ancestry
to the California of the Spanish/Mexican period.  I.e., their
ancestors almost certainly didn't live in what is now California
during the period of Spanish/Mexican ownership/occupation of the
region in the mid-19th century and before.

Huntington Park itself had a total population of only 526 people as
late as 1906.  See http://www.huntingtonpark.org/index.asp?NID=99 for
a history of the community and for this statistic.  The community was
in fact part of a large Mexican rancho during the Mexican period but
it's unlikely that the Lugo family's  (who owned the rancho)
descendants still live in what is now a very working-class, urban
community.

You'll see also at that site that the community proper (a town rather
than an agricultural region or part of a large land grant/rancho) and
its very Anglo name date to the very early years of the 20th century,
and the community was named for Pacific Electric Railway head Henry
Huntington.

The city was in fact predominately "Anglo" for a number of decades in
the 20th century, and is now "full of Mexicans" for now reason other
than the simple fact that Latino immigration to Southern California
has increased substantially since the mid-20th century and "inner-city
L.A. suburbs" like Huntington Park with their close-in locations,
access to industrial areas, proximity to the traditional Latino
stronghold of East Los Angeles, and relatively poor housing stock and
lack of desirability for even somewhat upwardly-mobile whites, are
some of the most heavily-Hispanic communities in the L.A. area, for
just those reasons above.

I just want to clarify one part of my comment.

In my comment "the area was in fact extremely sparsely populated, and
those that did live there were more likely to speak native languages
than Spanish," the "there" I'm referring to is California rather than
Huntington Park specifically. It sounds as if HP was part of a large
rancho owned by a Mexican family at the time of U.S. annexation, so I
imagine Spanish was the predominate language of what is now HP itself
in the 1840s, although perhaps only a very few number of people lived
there.  I stand by my comment about native Indian languages for the
state as a whole, however, in the 1840s, and certainly by my comment
that the Latino residents of today's HP can trace their ancestry to
places in what is now Mexico and points further south rather than to
what is now California.

Wikipedia's entry on Huntington Park
http://en.wikipedia.org/wiki/Huntington_Park%2C_California is nearly
perfect, and should answer your question much better than my attempts
above:

>>>> Named for prominent industrialist Henry Huntington, Huntington
Park was incorporated in 1906 as a streetcar suburb for workers in the
rapidly expanding industries to the southeast of downtown Los Angeles.
(To this day, about 30% of its residents work at factories in nearby
Vernon and Commerce. [1]) The stretch of Pacific Avenue in downtown
Huntington Park was a major commercial district serving the city's
largely working-class residents, as well as those of neighboring
cities such as Bell, Cudahy, and South Gate. As with most of the other
cities along the corridor stretching along the Los Angeles River to
the south and southeast of downtown Los Angeles, Huntington Park was
an almost exclusively white community during most of its history;
Alameda Street and Slauson Avenue, which were fiercely defended
segregation lines in the 1950s, separated it from black areas.

The changes that shaped Los Angeles from the late 1970s onward--the
decline of American manufacturing that began in the 1970s; the rapid
growth of newer suburbs in Orange County and the eastern San Gabriel
and western San Fernando valleys; the collapse of the aerospace and
defense industry at the end of the Cold War; and the implosion of the
Southern California real estate boom in the early 1990s--resulted in
the wholesale departure of virtually all of the white population of
Huntington Park by the mid-1990s. The vacuum was filled almost
entirely by two groups of Latinos: upwardly mobile families eager to
leave the barrios of East Los Angeles, and recent Mexican immigrants
(mostly undocumented) displaced by rapid economic changes in their
homeland. Today, Pacific Boulevard is once again a thriving commercial
strip, serving once again as a major retail center for working-class
residents of southeastern Los Angeles County--but unlike its previous
heyday of the 1930s, the signs along the avenue's storefronts are
virtually all in Spanish.<<<<<<

If you look at this site you will see that Huntington Park was founded
as a working class community, and still is, hence a likely place for
Hispanics to settle as they became established and home owners.

http://www.huntingtonparkcalifornia.com/local/cityinfo.html

http://www.city-data.com/city/Huntington-Park-California.html