you can possibly get from eating it. When you have a nett-zero energy gain from your food -there is no energy for anything else. You can't SPEND your ingenuity, innovativeness and resourcefullness on building better technology - because you need every bit of it just to survive every day.
Europeans on the other hand had wheat- one of the most energy rich food sources in the world. They were building grain stores, because they could actually in 3 months of labour provide enough food for the village for a year - leaving 9 months to spend on other pursuits beyond finding food.
Finally - Europe is laid out East-West in roughly the same set of latitudes. This is important - it means all of Europe has fairly similar climates. What you grow in one place, can be planted in another. Migration doesn't mean starting over with new resources. Travel is easier too - so ideas and technology spread faster. 

I highly reccomend giving Diamond a read. His chapters on Africa are enlightening stuff.