My guess is it’s to offset the lack of other benefits-- health care being a huge one-- that you lose when you sign up to be a gig worker, not a full-time worker.

And we already have different minimum wages for at least one industry: servers at restaurants. The economy isn’t going to collapse if we put gig workers in their own category, too.

The article answers your question.

Unlike most jobs, contract jobs are taxed more and require the worker to pay the out of pocket to operate. In the case of food delivery workers, this means the gas or electricity to run their vehicle and the maintenance costs for said vehicle.