Why did it take years to reveal the mystery to Paul?
It took years because Saul of Tarsus was steeped in Judaism and, being human, needed time to have his thinking and behavior cleansed. Also, grasping truth never before revealed naturally required time, and he had to be prepared and skilled before going to the Gentiles.
Les explains that people asked why, after Saul of Tarsus was converted on the road to Damascus and led down to Mount Sinai in Arabia for three years, it would take so long. One reason is that Saul was steeped in Judaism and was just as human as anyone else, so it probably took time to cleanse his thinking and behavior under Judaism. On the other hand, giving him a clear-cut understanding of things never revealed before would naturally take time. He also had to be so prepared that, when he returned from that Arabian experience, he was ready to go among the Gentile world and abject heathen idolaters, skilled in what he proclaimed. Les sees that as part and parcel of the three-year training time in Arabia, and Paul makes plain all of it was revealed to him and he was responsible to pass it out to the whole world.