I believe because he was the only Aposotle there at that time.
Peter was (supposed to have been) killed in Rome about the same time as Paul.
Given that the sarcophagos could turn out to be dated to hundreds of years after Paul's death, then it is highly likely we will never be able to prove it beyond all doubt.
Not that it will stop millions of people believing it to be him.
Interesting discovery nonetheless.
PAUL WAS BEHEADED 65-70
PETER WAS CRUCIFIED 80-85