I would assume it's because they don't want to. Either you don't have enough work in the city to support them yet, so they haven't moved, or they just don't plain like your city J/K., be patient, they'll move in as the work arrives. If you have the wrong kind of work (using high density industry, or high density commercial), they may not be qualified for, or want those jobs.
I'd just start with Low Resident, Low Commercial, Low Industry. It'll grow, you'll see.
By the way, I have a big city, but still use Low Res 1X1 plots. I really like the look of them, and make cool little suburb neighbourhoods and stuff