Really? That seems especially odd to me. Why did he wait 20 years to start writing about Jesus? He was alive and adult in 30 AD, wasn't he?
We don't know for sure he waited. In the opinion of Crossan and Borg, Paul, after joining the Jesus movement, went on a mission to Arabia (as Paul states that he did go to Arabia; however, he doesn't mention much about it). The mission was seemingly a failure, and thus, there would be no reason to expect writings to be preserved there.
We know that Paul wrote more than what we actually have. He tells us this by mentioning letters we no longer have. So it is very possible that Paul wrote before 49 C.E.
If Paul converted in 33 to 36, I cannot imagine how his writings could be empty of Jesus reports. Is that what you believe? That a man with Paul's passion for Jesus, in touch with men who lived and worked with Jesus, would not give us a single report from those men about Jesus himself?
That's beyond belief. His writings would be full of stories about his Master's worldy work.
I really don't see how anyone with a rudimentary understanding of human nature could argue otherwise, but I'll be glad to listen if you have a reasonable explanation.
Why didn't Paul speak copiously of Jesus' ministry on earth?
You have to understand the purpose of his letters. The letters were not evangelical tools. He wasn't trying to spread the message of Jesus with his letters. He had already done that previously.
The people Paul was writing to had already converted to the Jesus movement. He had already been in their cities, preached to them, and gave them the background that they would need in order to continue when he was gone.
Later, he would receive letters, or sometimes reports from these churches he already founded, asking either questions, or stating problems they were having. Paul would then address those questions and problems.
All we can gleam from the fact that he wrote little about Jesus is that people were not having a problem with his life. No one was questioning aspects of the life of Jesus. So there was no reason to really address that.
More so, much of the focus was not on the life of Jesus, but on the death and resurrection. It was that which was most important to them.
One tossaway comment about meeting the brother of Jesus... and that's enough for you.
It seems awfully curious to me.
It is more than one comment though. He states specifically once, that he met the brother of Jesus. He later mentions this same James on various occasions.