The linked WaPo article has a little more detail. Here’s the core of it:

DoorDash and Grubhub argue in a joint lawsuit that the rule is based on “inherently biased and unreliable survey data” and would hurt delivery drivers rather than help them.

If the rule goes through, the companies say, the increased costs would be passed on to consumers, which would result in fewer customers for delivery workers to deliver to and “injure its goodwill within the industry,” according to court filings.

In a separate lawsuit, Uber said the increased minimum wage would hurt local restaurants because the higher costs could dissuade customers from ordering.