Why do some people insist Catholics aren't Christians?

Mainly because they have no knowledge of the history of Christianity. Plus they desperately need a justification for the 16th Century rebellion against the Catholic Church that spawned their own unauthorized manmade tradition. They know their tradition exists in open defiance of the stated will of Christ "that they all may be ONE". But they hope that maybe, just maybe, if the Catholic Church can be shown to be BAD enough, then maybe that rebellion and the resulting thousands of conflicting manmade denominations and all of the conflicting and therefore false teachings can somehow still be justified. And what could be more BAD than "not even Christian? Of course it is a bit difficult to explain how Christian churches branched off from a non-Christian Church. But that is really the least of their problems.