It is important to remember that in this novel, the gang of Greasers of which Ponyboy is a part is actually a family. As Ponyboy and his brothers are orphans, Darry, Sodapops and their friends become substitute "parents" for Ponyboy, as he is the smallest and youngest of the gang. In addition, Darry takes his responsibility as substitute father very seriously, because he wants Ponyboy to have the kind of life that the death of their parents made it impossible for him to have. Thus it is that the gang look out for Ponyboy and feel a real sense of responsibility for him. Note, for example, how Darry responds to Ponyboy's request to be allowed to fight in the rumble in Chapter 9:
"Well"--Darry gave in--"I guess you can. But be careful and if you get in a jam, holler and I'll get you out."
Darry does not treat Sodapops in the same way, because he is able to "use his head," whereas clearly Darry feels greater responsibility towards his younger brother.
He's young, good looking, athletic, smart. He is what they wish they were. He comes as a great package.
Unfortunately, he comes from the wrong side of the tracks. They will do anything to keep him safe as they see him as "the hope" of the group.