The de-rate the engines due to longevity. A commercial truck typically see's many more and harder miles than a pickup typically would. Engine output is correlated to how long an engine lasts, so a commercial truck that would see 100K+miles easily needs to be able to last for customer expectations vs. a pickup that may see 50-80K and who's driver is more concerned with performance.
also, the more displacement an engine has the less it has to work, thats why your seeing 15Liter engines on the big trucks. A 6.0L with 400hp works much harder than an 8L with 400hp.