Why were the states between California and Texas admitted in such a neat, linear sequence?

The 48 contiguous states west of the Mississippi were admitted in this order: MO, AR, IA, TX, KS, NE, CO, ND, SD, MT, WA, ID, OK, NM, UT, AZ. 17 states over 20 years. Why were they admitted in this order? Why wasn't there more disorder in this sequence?