Different citation styles support different areas of study
Over time, different disciplines in academia have come to prefer certain citation styles over others. For instance, MLA format is widely used in the humanities, since the style is well-suited to citing literature and archival sources. Conversely, APA format is widely used in the social sciences, since the style performs well with quantitative studies and analysis.
Certain styles can have multiple citing systems
One reason that it appears that the number of citation styles is so vast is because of the possibility of different formatting systems within the same style. For example, Chicago style actually has two different sub-styles: author-date and footnote-bibliography. The footnote-bibliography system can be commonly found in humanities courses, whereas the author-date system has a more broad application.
Different papers appeal to different types of audiences
One reason that we can’t simply have one uniform citation style is that each academic paper can appeal to a vastly different audience than another. Since researchers working in different areas are writing for a specific audience, such as a science professor or fellow scientist, they want to make sure that their citations clarify information and sources that are most highly valued in their subject area. A writer in the social sciences would be more likely to cite a scholarly article than a writer in the humanities focusing on archival materials, so their citation system should match this.
There is an element of tradition in citation styles
One of the many reasons that there are so many citation styles is simply that many have been in publication for generations. The Chicago Manual of style, for instance, traces its roots back to 1906. That means that writers and researchers in these disciplines have come to rely on a specific citation style when creating their work, and could be reluctant to try a different one.