According to smartphonematters.com Straight Talk uses the four major networks Verizon, AT&T, Sprint and T-Mobile but they don’t offer all four networks in every area.
In some cases the Verizon network might not have coverage in your area and Straight Talk is unable to offer it. In other cases Verizon might be available in your area but Straight Talk doesn’t offer it there.
When you sign up for Straight Talk they use your ZIP code to check what networks are available in your area.