Why could Darry have been a Soc if not for the gang?

Darry could have been a soc because he met all the physical and mental requirements to be one. He was intelligent, athletic, and attractive. However, his obligations to his family and the gang held him back from capitalizing on those qualities.
Socs — short for socials — are a group of wealthy teens who have material things that the other people lack. One of the things that Ponyboy says they have are "futures". When he thinks that Darry could have been a soc, he's thinking that he could have had a future if he wasn't held down by his obligations to the gang and his family.
One of the things that holds Darry back is that he's responsible for his brothers. Their parents died and he had to take over the household. Before they died, though, he was on a good track with athletics and school.

In Chapter 8, Two-Bit, Dally, and Ponyboy are on their way to meet up with the other Greasers before the rumble and Pony tells Two-Bit not to tell Darry he's sick. Two-Bit comments on Darry's strict personality before saying,

"You know, the only thing that keeps Darry from bein' a Soc is us" (Hinton, 107).

Pony thinks to himself that he is aware Darry could have been a Soc because he is too smart to be a Greaser. Unlike the other Greasers, Darry is intelligent, athletic, and cuts his hair. Even Darry's old teammates are Soc members. Paul Holden used to play tailback on Darry's high school football team and the only thing separating them is their different situations. Once Darry's parents passed away, he was forced to turn down an athletic scholarship and work two jobs. Similar to his youngest brother, Darry earned good grades in high school, does not have a criminal record, and does not really fit in with the other Greasers. Given Darry's physical description, athletic ability, personal style, and intelligence, one can assume Darry could have been a Soc member.