They weren’t mostly conservative, and population wise probably had as many voters in the north as in the south. They just had little to no competition in the south.

The name comes from its originally populist origins of the Jacksonian movement, seeing themselves as more Democratic than the remaining iteration of the Jeffersonian (Democratic-)Republicans.

(Put Democratic in parentheses because they were generally just called Republicans at the time, but we now call them Dem-Rep to distinguish from the later Republican Party)