The real reason is political will. Democrats and Republican don’t want another state that will give the other party two more Senate seats.
For instance, if you made DC a state there is a 99.9% probability it would have two Democratic Senators. So to get Republicans to go along with the idea you’d need another state that was 99.9% likely to produce two Republican Senators.
DC is out of the running also because it’s populaton once was over 800,000 people and it was actually bigger than a lot of western states. Now that is not the case. DC has just under 600,000 people and is only bigger than Wyoming today
When AL and HA were admitted this was the case. It was assumed one would be basically Democrtic and one Republican. In 1954 then President Eisenhower gave a State of the Union address, and requested the immediate admission of Hawaii into the Union but did not mention Alaska.
It was thought Hawaii was going to be Republican and Alaska Democratic. However the reverse turned out to be true.
There are no minimum population restrictions anywhere. Congress has set various minimums throughout history from 60,000 to over 150,000. Congress can set them as well as unset them
And even then it’s doubtful any minumum would pass a test, as Nevada had only about 40,000 when it was admitted and the law said 60,000. So it’s clear that law stating minimum population is not enforceable. Even though Nevada eventually passed 60,000 by 1880 by 1900 it was back down to 42,000 people. (There was some talk of taking away Nevada’s statehood, but that is all it was talk, that could never have been done.)
State can split to form new states, as long as the state itself concents and the Congress does to. This would never happen as the Democrats and Republicans would have to be 100% certain the balance would be there.
Two states can join together to form a state, as long as both states and Congress consents. Again very unlikely as they would go form 4 Senators to 2 Senators.
And parts of two states can join t form a new third state. The most talk about example of this is "The State of Jefferson (proposed). This would be Northern California and Southern Oregon
The real reason is there is no poltical will.