Perhaps Paul's message differed so much from Jesus's because Paul knew next to nothing about the historical Jesus (assuming there was such an historical figure)?

You might be interested in my hard-hitting article on the example Paul set for Christian apologists:

https://medium.com/interfaith-now/pauls-epistles-epitomes-of-christian-obnoxiousness-b38de481a398?sk=a95f5463832f23cc5d713d34139f5396