Why is being called a Christian as a Catholic so disputed?

Because everyone thinks they are correct and do not wish to be ociated with anyone that disagrees with them. There is a distinct difference between the catholic faith and Christianity as a whole as the world understands it. Catholics for the most part are not Christian as Christian (baptist, pentecostal, methodists, etc) understand Christianity to be. Catholics, in their belief, suppose all others to be wrong as well. Basically, you can't win but stop worrying about offending anyone. That is not your problem, but theirs.