Why is the Socs always written with a capital letter in the book and the greasers with a small one?

That's a great question.

As
you know, capital letters are used for proper nouns -- the names of things -- while
lower case letters are used for common nouns.

To me, what
is going on here is that the word "Soc" is always used in this book to mean a member of
that particular gang.  So that means it's a proper noun just like it would be proper to
call someone from the Republican Party a Republican.

By
contrast, the word "greaser" is being used to describe a general type of person.  The
author uses it to mean someone who dresses and wears their hair a certain way -- not
necessarily a member of the gang.

I think that it is also
possible that the author is doing this to make a point.  She may be doing it to give the
idea that the Socs are important and that the greasers are not.