Why was Paul the only apostle who clearly taught the pre-trib rapture?
Paul was the only Apostle taught by Jesus after the Resurrection. All the others were taught during His earthly ministry, while the offer of the Kingdom was still open to Israel. Since the Lord didn’t teach about the Rapture during His time on Earth, it’s understandable that His disciples didn’t teach it either.
In Galatians 1:10-24 where Paul explains how he gained his knowledge, he hints that Jesus taught him personally during the 3 years he spent at Mt. Sinai in Arabia after his conversion. Later, he also visited God’s Throne (2 Cor 12:2) and learned things there he wasn’t allowed to fully disclose.
God had clear strategic reasons for not revealing the rapture to anyone before the cross. In 1 Cor 2:6-10 Paul wrote that if Satan had known what the Lord had in store for the Church, he’d have done everything in his power to prevent the crucifixion. Paul called it part of God’s secret wisdom there and in Ephesians 3:10. In addition, although God knew in advance that Israel would reject the Kingdom, He couldn’t disclose His plans for the Church until Israel made their rejection official. It was not until 51AD that God finally allowed Paul to reveal the doctrine of the pre-tribulation rapture in his first letter to the Thessalonians (1 Thes 1:10, 1 Thes 4:16-17, 1 Thes. 5:9).