Different citation styles are mainly used by distinct academic disciplines. Each organization that has created a citation style guide for their academic discipline or profession tends to emphasize different elements of a citation in order to make it easier to both create and interpret what is being cited.
One example: humanities researchers aren't often concerned with the date something was published. A book or article about Homer, for example, published in the 1950s can be just as valuable as one created today. The MLA citation style doesn't put the date near the front of the citation, because it's not as important. However, in the sciences, date can be very important, as knowledge about new biomedical techniques or research into curing a disease, for example, can supercede previous research. Therefore, dates are placed near the front of the citation in scientific citation styles.
The differences between citation styles may seem overly picky, but if you dig a bit deeper, there is usually a reason for why they are structured the way they are.
