The DOT places more restrictions on operators of heavy trucks than on lighter trucks. Why do you think this is?

Only the states can regulate drivers of light trucks.

The drivers of heavy trucks go on long hauls, and therefore are most likely to engage in smuggling and related activities that might pose a national security risk.
The DOT does not actually place more restrictions on operators of heavy trucks.
The DOT is tasked with protecting the safety of people on the roadways, and long hauls pose more such risks.