Ehrman ignored the overall explanation in Genesis and went to specific examples in Job, the Prophets, and the Apocalypse.  Having ignored the basics in Genesis, it’s not surprising that Ehrman came up with the wrong answer.

‘Depending on what part of Job you read, you get one set of answers. If you read the Prophets, you get a different set of answers. If you read apocalyptic literature, you get still a different set of answers … Finally, because I became dissatisfied with all the conventional answers, I decided that I couldn’t believe in a God who was in any way intervening in this world, given the state of things. So that’s why I ended up losing my faith.’  Ehrman

Death is the result of human sin.  That is the story of Genesis.

A more intelligent question might be this: why the all-knowing God allowed sin.  He knew that Adam and Eve would sin?

The answer is that God had his purpose, and He knows exactly what He is doing.  He intends to bring the creation, not to what it was, but to something that it could have never attained had it never fallen away. 

God gave His Son, Jesus, as the only way to be redeemed from sin.  His plan is not fully revealed.  We do know, however, much about His plan.  God has revealed much of His plan in Scripture, but even the part that He has revealed is beyond the ability of many Christians to accept of grasp.  Our finite minds would certainly not be able to comprehend the fullness of what God has in store for us.  We can see that the splendor of the new heavens and the new earth far exceed the splendor of the first.