For most of the 20th century, Missouri's Democrats were in St. Louis, Kansas City and the rural Southern part of the state.
The rural Southern party of the state swung Republican with the rest of the South.
St. Louis and Kansas City have been stagnating or declining in population for the past several decades and exercise less and less clout within the state.
This plus I think it would be easy to gerrymander because all the democratic voters are packed into certain areas whereas republican voters are spread across all of rural Missouri.
