The reason is that a diesel is not "throttled." A gas engine regulates power by varying the amount of air and thus fuel that enters the engine. With the throttle closed or near closed, or with the engine at higher RPM, it cannot get all the air it wants past the throttle, that creates vacuum.

A diesel on the other hand has no throttle restricting the air. The power is regulated by the amount of fuel that is injected into the cylinders. Since it gets a free, easy draw of air, there is very little vacuum created.

Imagine this. Put a vacuum guage near the open end of a vacuum cleaner hose. Since there is no restriction, very little if any vacuum will be measured. Now cover the end of the hose and watch the vacuum reading increase.

Hope this helps,