That question is like asking why there are many different engine options for a class 8 truck. They are commercial trucks with fleet use being one of the markets that is targeted. If you owned a fleet of trucks would you want to pay for service people that need to be able to work on more than one brand of engine? The answer is no. You would want everything you owned to be the same or as close to the same as possible. Cummins's engine sales is base on reliability and customer following of their larger engine lines. However, they are atleast one generation of technology behind International (Ford) and Isuzu (GM). The key to the reason the Ford engine is so much better is its new variable vane turbo. GM will have a similair set up on their Duramax's within the next year.