Real simple. Despite early radicalism up to the FDR administration, once the working rural classes got paid enough that they could afford a simple 3 bedroom home with a couple of kids on a single wage earner's take home pay, they considered themselves middle class. That was their definition of being middle class at least. So culturally, this working "middle class" associated the 'follow the rules, pay your taxes, do a day's work for a day's pay, etc' with what the Republican party espoused. Especially since at the time, a great many of the urban centers were Democratic strongholds and suffered a whole host of social ills like drugs, crime and single-parent households. So culturally, Republicans got associated with positive cultural traits while the Democratic party got associated with what looked like a lot of cultural traits that held negative results for people's lives.
The Democratic party also did a long march into Academia while abandoning the blue collar voters that had been their historical strength. Because deindustrialization of the economy and other globalization pressures like the rise of finance meant that the rural blue collar voters were a dead-end block and couldn't provide the funding.
Then you add the progression of Political Correctness to Wokeness, and the attitude that dismissed the rural voters as 'Deplorables', and those voters know that the Republicans do not have their economic interests at heart, but the Republicans do not actively despise them.