Congress was following a general scheme of trying to make the new states created out of the Louisiana Purchase as equal as possible, approximately three degrees in height and seven in width. An enormous State of Dakota would have been both conspicuous and harder to administer. According to Mark Stein's How the States Got Their Shapes, Pres. Harrison even shuffled the bills for North and South Dakota so no one would know which one he signed first (and thus which was admitted to the union first).

To add on, could any parallels be drawn to the Carolinas?