Look at the text of the Nicene Creed. There is a phrase that reads, depending on the translation, something like "one holy catholic apostolic church". That is the core of the issue.

All mainstream Christian denominations are branches of that original church. Some may see each other heretical, but they still acknowledge the common heritage. If you now have a religion that takes some aspects of Christianity, adds something of its own, and rejects the part all those schismatic branches agree on, it does not look like you are a branch of that "one holy catholic apostolic church".

The same applies to Jehovah's Witnesses.