“I know you’re trying to find out the reason people aren’t doing it,” Mr. Grayson said on a recent afternoon. “I’m going to tell you. People are trying to take care of their household. You don’t have much time in the day.” The slowdown in vaccinations across the country has often been attributed to a blend of misinformation and mistrust among Americans known as “vaccine hesitancy.” But Mr. Grayson belongs to an overlooked but sizable group whose reasons for remaining unvaccinated are not about opposition to the shots or even skepticism about them. According to a new U.S. census estimate, some 30 million American adults who are open to getting a Covid vaccine have not yet managed to actually do so. Their ranks are larger than the hesitant — more than 28 million who said they would probably or definitely not get vaccinated, and than the 16 million who said they were unsure. And this month, as the Biden administration set a goal of 70 percent of adults getting at least one dose by July 4, they became an official new focus of the nation’s mass vaccination campaign.