It's certainly safer for the environment in general in terms of oil spills but the communities that would suffer from truck or trail spills are different from the communities that would suffer from pipeline spills. And since the former don't get to sue but the later do there's a certain pressure against pipelines in general.

But on the other hand pipelines are more cost effective than other means of transport. That means the oil gets to market more cheaply and that will tend to decrease the price of oil, meaning more is used, meaning more CO2 in the air. Raising the price by taxing oil would mean that we would get the double benefit of less oil use and government money to pay down the debt or whatever but that's harder to do politically than just block pipelines.