One is that Republicans oppose public -anything-. Public education, public health insurance, public libraries, etc. They don't believe government works, or even that it -can- work. And when they are in control they have no incentive to make it work.
The other is that Republicans have an agenda that is all about concentrating wealth and giving corporations more power over our lives. Public transportation doesn't make huge profits for corporations. They would rather protect and prosper corporations, because that's where their campaign funds come from.
One of the differences between liberals and conservatives is their view of capitalism, of the pursuit of profits. Liberals believe in capitalism but they see the pursuit of profit as a means to an end, the end being a rise in the standard of living for everyone. Conservatives see profit as the end in itself. GW Bush, for instance, bragged for the first six years of his presidency that the economy was growing rapidly. Which it was, on paper, but 95% of Americans were losing ground because all these gains were going to a tiny sliver of economic elites, less than 1% of the population. We were all getting poorer so a few gazillionaires could get richer. That, to Republicans, is -success-. Public transportation doesn't serve that goal.