Identitarian is someone who believes in identity. If something ends in arian, it's a kind of person. Vegetarian is another example. "-ism"s are ideologies like capitalism or communism or socialism. Vegetarianism is the ideology that vegetarians follow. In the case of capitalism, the people who follow it are called capitalists., have communists and socialists as well. We could have called vegetarians vegetists and their ideology, vegetism but for some reason, we went with the other one. It's two suffixes that sort of mean the same thing.
โism is a suffix added to the end of a word to indicate that the word represents a specific practice, system, or philosophy. Often these practices, systems, or philosophies are political ideologies or artistic movements
Things just often end up being more or less specific than we think they are at first. Then there are words so specific they feel useless and not enough words at other times to describe what we want. English isn't as bad as other languages when it comes to having too many words but sometimes it feels silly.