Why does the word "atheism" exist? There is no such word for people who disbelieve in astrology or Santa Claus or dragons. So, why does the word exist? Is it because most societies today and in history have marginalized non-religious people? I apologize if this question is not appropriate for Philosophy stack exchange. If it is not, it should be migrated to an appropriate SE.
Words become a part of standard language because they are useful to a lot of people. There is very little use for referring to the class of people who disbelieve in astrology or Santa Claus or dragons; there are a lot of uses for referring to the class of people who disbelieve in God.
You are probably working under some sort of implicit assumption that because language is a social phenomenon that the evolution of language has an element of deliberation. This is not the case. Think of it like an ant trail. No ant or committee of ants decides to make a long line of foragers to your trash can; the situation develops organically as the individual actions of a lot of different ants.
Language works the same way, as do economic and cultural changes. These large social movements generally evolve without conscious control of any individual or group. Of course, certain individuals or groups are more influential than others; the owner of the New York times has a lot more influence on the direction of language than, say, your local butcher, but even he can't control it entirely. No one can.
Governments may be able to somewhat control the direction of major social changes, but even they fail eventually. Witness the inability of the Academie francaise to prevent imports of loan words. More spectacularly, note the various attempts by socialist and communist governments to control the economy and culture. No matter how many people they tortured, imprisoned, and killed (in some cases it was millions), they almost all eventually failed. Others, like North Korea and China are still torturing, imprisoning, and killing people in an endless horror show to keep their regime in power. Social change is just very difficult to control.
So if you are looking for a reason in the sense of "why did someone decide to make 'atheism' a word", there isn't any such reason. It was just an evolution of language.
ATHÉE n. xvie siècle. Emprunté, par l'intermédiaire du latin chrétien atheos, « qui ne croit pas en Dieu », du grec atheos, « qui ne croit pas aux dieux ».
According to this current Académie Française definition, the word "athée" apparently only started to be used (in French) in the 16th century, borrowed through the Christian Latin "atheos", meaning "who doesn't believe in God" to the Greek word "atheos", meaning "who doesn't believe in gods". People make up words, sometimes deliberately, as obviously in this case, because they need or just want to name something. It should be noticed that this definition doesn't do justice to the fact that it wears its semantics on its sleeve, as it were, because rather than "who doesn't believe in God", the meaning of "atheist" is on the face of it just "godless". And, "godless", which also means what it says, is also used in English as the slur "Lacking reverence or immoral". I expect this is the original intention in the creation of the word "atheos" in Greek, its use in Christian Latin, and its use in French or English. I wasn't there at the time but this is what seems to be the most plausible interpretation.