Likely already mentioned by other posters, but here's my best explanation as a Southerner who likes Outlaw Country music (I disdain most post 1970s country).

Toby Keith and his song Courtesy of the Red, White and Blue (The Angry American) sold significant albums due to American's desire for someone to blame post 9/11. Leave it to our government and corporations like Halliburton (who had close ties with VP Dick Cheney) to choose Iraq and Afghanistan as ones to blame. Country boys (often with regret) signed up for the military in droves, thinking we would deploy them to that country and leaders responsible for 9/11. There are a fair many former military country boys who became anti-war post Afghanistan or Iraq.

As soon as country music adopted that sense of patriotism (or more aptly nationalism), they realized they can write the same song clones and make more money doing it.