Referring to of course the Dakotas, Wyoming, Idaho, etc. I guess the obvious answer would be that maybe they wouldn't want to group them together because the territories being too big. But given how controversial the large relative representative power per person of these states is today, you'd think there would have been at least as much of a controversy during the 1800s when statehood was at issue, especially given how they'd dilute the power of the then-existing states.
But when I read about, for example, why there's two Dakotas, it seems like the Dakotans just didn't like each other and there was little thought given to the issue of how powerful these two new states would be. Is it so simple as they just didn't think about it? Or was there some other incentive to shift power to these states?
Edit: Just want to specify that some prior questions deal with the question of why these states exist, but I don't see any that deal specifically with whether there was any controversy of how they would shift the power balance and that's mainly what I'm asking about.
There are several factors that I believe contributed to the number/size of states in this region.
I think the first place to start is the Louisana Purchase which encompasses many I of the states that I believe you are referring to. This specifically leaves out California, Texas, Arizona, New Mexico, and Utah (to some extent) as those states/regions had their own origins and reasons for being the sizes they are. This helps to narrow down the area/regions we are talking about. I think it is also fairly safe to set aside Oregon and Washington as well since their formations came about due to immigration and some conflict/compromise with Britain. This leaves us with states West of the Mississippi, the Northern 2/3 of the Rockies and Great Basin East of the Sierras.
Next, the lens of slave state vs free state shapes and colors states/territories through the Civil War. Louisiana as a state added a key port to the country and added a slave state that could be said offset either Ohio or Indiana for the balance of power in the US Senate. I would argue that Arkansas, Missouri, Iowa, Wisconsin, and Minnesota were all added as states to help maintain balance in the Senate.
By 1858, the balance and tide is shifting. There are 17 Free states vs 15 slave states. The West (now the MidWest), is growing faster in population and that population is also increasingly ani-slavery. That West could be counted on by the Slavery forces as being at least somewhat neutral or even an ally in terms of national politics, but that was beginning to change.
Oregon, Kansas, and Nebraska (to a lesser extent) had reached the threshold for becoming States and territories and the balance of power now threatened to shift even further away from the Slave States. The Kansas/Nebraska act was an attempt to let their namesake territories choose for themselves and it was hoped that they would split and Kansas would join as a slave territory and state to help continue the balance.
So that covers the states and territories pre-Civil War. You already mentioned the split in the Dakota Territories. Utah was it's own case due to the Morman settlements. Nevada, Arizona, New Mexico, Colorado, Wyoming, Montana, and Idaho are more or less the rest of the states and territories added after. Each of them have an interesting story, path, and journey to statehood.
While each state had a unique journey, the primary designation and formation of a state revolved around establishments of state/territorial constitutions. The bar was set at having a workable local and state/territorial wide governance that the people in said areas would agree upon in a series of elections. Congress passed many acts allowing territories to organize and form governments and the many disputes, quasi-wars, bribes, and outlawery really make for fun and entertaining reads.
I believe that the last piece and reason why there were so many lower populations states added at the end of the formation of the 48 states came down to how the country viewed Manifest Destiniy in the formation of a country from sea to shining sea. Most if not all of these new additions had their economies revolve around farming and mining (or both). Additionally, while cities were certainly growing with the power of the industrial revolution, the mining and agricultural sectors still required a large population of human labors to feed that growth. Even in the early 20th century it still seemed very likely that many of these new states could possibly grow into larger population centers than the old states. A quick and easy example of this is the 1900 census. California despite it's size and resources just barely passes Kansas to move up to the 21st most populus state and that still puts it ahead of solid 1/3 of the orginal states. The idea of granting statedhood and giving equal footing to the new states as the old goes back to the founding of the Constitution. In the run up to the Civil War adding states often lent the idea that it was adding power to the slave or free state cause, not dilluting it. After the war, the idea of adding states soldified as part national pride and part out of necissity to gain access to the resources needed to fuel the economy.
It should come as no surprise that many of these lower population states now have many "ghost towns" that were once growing centers of population and even industry. However, once it became cheaper and less labor intesive to extract the resources in those areas the need for that higher population made it difficult to sustain those towns and so they now slowly fade into the landscape.
A quicker summation would probably be: States were added to add power to slave or free state factions until the Civil War, post Civil War states were added to gain agrarian or mineral resources.