They’re limited to a specific amount of time to pose questions in order to get through all the representatives that want to ask questions. Although, as pointed out, any representative can yield their time to another member or the Chair.
Cummings is either ignorant...an Obama Sycophant or deceptively political when he says the ACA has helped millions of people, and “YOU GRUBER, have given the anti ACA guys...the ammo they needed.” to paraphrase. Clearly, the ACA is a dangerous piece of legislation, put together buy people like Gruber.