If you're using MLA, APA or Turabian, one of the best places for in-depth citation help is the Purdue Online Writing Center (OWL).
If there were only one universal format for citations, it could make it difficult for some researchers to find the information that is most important to them about a source. For example, social sciences tend to use current research, so the publication date of a source is very important. For this reason, people in the social sciences tend to use APA style; APA style puts the date before all other information, aside from the author’s name, which makes it much easier for researchers in that field to find valid, up-to-date information.
Humanities fields such as literature, on the other hand, do not rely so heavily on dates, and instead focus on the title of the work and all the people/organizations involved in creating it. For this reason, the humanities tend to use MLA style, as this style gives precedence to that information.
Chicago style is the most flexible of the three styles listed, and it has two subtypes: Notes and Bibliography, and Author-Date (University of Chicago, 2010). The Notes and Bibliography subtype is more commonly used in the humanities (when the MLA style is not being used) because it focuses more on the bibliographic information of a source such as the author’s name(s), the title, and the publishing information (University of Chicago, 2010). Overall, it is fairly similar to MLA format, but instead of parenthetical in-text citations, it uses footnotes.
The Author-Date subtype of the Chicago style is more used in the “physical, natural, and social sciences,” according to the official Chicago Manual of Style website (University of Chicago, 2010). In this subtype, the author’s name and date are once again prioritized. This version of Chicago is also quite similar to APA, and it also uses parenthetical in-text citations like APA (whereas the Author-Date subtype does not).
It may seem silly to some people to have such similar citation styles, yet still not fuse them together to form one universal citation format. However, the slight rearrangement of information between each style is actually very important: it makes research much easier for those who prioritize different information. Therefore, although these formats are similar in many aspects, ultimately the little differences between each one are important for the fields that use them.