First: in the political-system of the US it is very hard for a third party to establish itself. Basically you have a two-party-system. So for everyone that is not happy with the democrats, the republicans are the logical alternative.
Then, both parties have basically the same economical recipes. So I can understand many people elect based on other considerations than economy and personal wealth.
The republicans have the big plus, that they are perceived as more fundamentalist christians. Poor people can also be hardcore-christians. So if you against gay-marriage and abortion and in favor of death-penalty and generally a more christian lifestyle, you are more likely elect republicans.
Also the republicans are perceived as more nationalistic. So if you want the US as #1 in the world (although neither republicans nor democrats can stop china from taking this position) and prefer solving conflicts with military force, you probably elect republicans.
Note, that I said in both points before, that they are perceived in this way.







