It's quite simple; net neutrality represents regulations on business. Republicans are anti-(business)-regulation, believing that a completely free market will deliver the best result to consumers. Of course, in this case that disregards the reality of 80 years of telecom monopolies (and derivative companies thereof,) i.e. mammoth established players with a history of playing dirty to stomp all others out of the marketplace; it is possible that in a blank slate scenario that regulation-less competition would lead to a good result for consumers, but even that is not a foregone outcome. In short, market idealism.