Why are Paul's writings in the Bible?

[LinuxGal]

The Jerusalem Church was destroyed in 70 CE [1], Shimon (the Lord's other brother) took the remnant over the Jordan[2], and only Paul's network of Gentile churches was left standing to write the histories and define orthodoxy[3].

And the evidence for that is what ?

[1] Flavius Josephus, The Jewish War
[2] Eusebius of Caesarea, Church History, Book III, ch. 11.
[3] Bart D. Ehrman (2002). "19: The Rise Of Early Christian Orthodoxy". Lost Christianities.