Money.

seriously. Prince George's county is, relatively speaking, flat broke. Polling places may be cheap, and so are volunteers, but the machines these days aren't. additionally, there's a lot of bureaucracy involved in redistricting things (and a lot of politics, to boot), and that also costs money and time that the county simply hasn't (or won't) allocate.

it's the sort of thing a county begs the state for money to pay for, but the whole state is like that.