Geartrains are great for longevity but can pose problems for noise. Automotive manufacturers keep that in high consideration these days. Quiet engines, especially diesels are a priority for many applications. Using a belt is a simple solution.
Actually a much cheaper solution. I don't think noise is a consideration with a diesel engine in this instance. Timing belts became popular in the 70's and 80's because single and double row chains would "grow" as they got to operating temperature. This became an issue for emissions compliance. Timing belts offered more consistent valve timing regardless of temperature, until they broke. Current triplex type chains and variable valve timing makes this a non-issue, but are more expensive to manufacture.
