Chevy ranks where they do because of the consistency, durability, strength, power, and reliability of both the Duramax and Vortec. Duramax is Chevy's diesel made by Isuzu. The Duramax has to be by far the second best diesel today. It has proven to last, and outperform everything besides the Cummins. 

Chevy gas trucks have a vortec engine. By far this is the best gas engine available. Along with great fuel economy, the vortec still pulls a great amount, and is proven to last a long time with fault.

Due to the great balance of the diesel and gas, that's why Chevy places second.