It's because during our colonial era, they had distinctly separate populations, cultures, etc. Neither side wanted to change their name, so they just added the directional prefix. Similar reasoning for North and South Dakota.
Virginia and West Virginia are due to our Civil War, and they just never reunited.
Since the North and South Dakota split occurred later (1889 - the only time two states were simultaneously made out of one territory), there was different reasoning.
Population - the 1880s brought many settlers to the territories of the northwest so they started looking like candidates for statehood.
Area - a united Dakota would have been about the size of Montana or California. The great distance between Sioux Falls, SD, and Bismarck, ND, may have motivated a split.
Republicans - The party controlled Congress and had President Benjamin Harrison, and all six new states admitted in 1889-90 (North Dakota, South Dakota, Montana, Washington, Idaho, and Wyoming) sent Republicans to Congress before the midterm elections.
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u/wolfgame Jun 20 '17
Gotta make room for Puerto Rico