
If you want the actual explanation, Republican politics tend to focus primarily on rural whites. Rural areas tend to be pretty poor. Furthermore, Republican politics are especially concentrated in the American South, due to a strategy of republicans exploiting white southern racism for electoral success. The South is very poor, due to the legacy of the civil war and a historically sluggish economy primarily reliant on cash crop agriculture.
There’s also a cycle from Republican policy. Republican policy tends to focus on minimal taxes, and cutting spending like education. This means that the education systems in red states are worse, so things stay shitty. They favor tough on crime policies which worsen the status of their communities, they try to minimize social services as much as possible, and their ideology actively discourages educational advancement, as that makes people more liberal.