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.