Why did Paul focus only on the risen Christ and not on Jesus' earthly ministry?
Paul focused only on the risen Christ because, as he said, though he once knew Christ in the flesh, after the resurrection he no longer regards Him that way. The power of the resurrection is all Paul knows; Christ's earthly ministry must be behind us. That is the reason supplied.
Les explains that Paul never concerns himself with Christ's earthly ministry; he concerns himself only with Christ after His resurrection. Paul acknowledges that he knew Christ in the flesh and likely had witnessed all Jesus had done throughout the land of Israel, but henceforth, since His resurrection, he says we know Him as such no more. The power of the resurrection is all that Paul knows, and the earthly ministry has to be behind us. Les also points to 2 Corinthians 13:4, where Christ was crucified through weakness—a reference to His earthly ministry of meekness and lowliness, having no place to lay His head.