Considering it for a few minutes, I attribute in part to three factors.
1. When religious authorities convened at the Council of Nicea, 325 AD, they chose thirteen of Paul's epistles/letters to compose nearly half of the New Testament.
2. Paul's missionary zeal, and his willingness to teach the gospel to gentile/pagan nations. He was willing to put in the work, and go into all nations whether they were receptive or very hostile of his teachings. The other apostles only wanted to teach the gospel to Jews.
3. Third, and the most important, I believe it was divinely inspired. Paul/Saul's christophany experience on the Damascus road, the protection and numerous times the Lord rescued/delivered him from harrowing circumstances, his zeal to teach the gospel at risk of great harm, even death. Also, his eventual murder which led to his martyrdom in christianity.