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.
Part of the reason for capitalizing Socs might have to do with their view of everyone around them, particularly the greasers. The Socs are the high on the teenage social ladder and view themselves as such. They are "cool kids" from the wealthier part of town, who drive nice cars, wear nice clothes, and basically disrespect anyone else they come into contact with. They are aloof and pretentious; they look down upon all others, especially the greasers who possess none of the luxuries as do the Socs. Perhaps the capital letter helps to communicate this idea for the Socs while the lowercase greasers better fits their perceived place on the social ladder.