Why do so many publication venues have their own citation styles?
It is a matter of history, technology, and personal preferences. Different citation styles have arisen in different times to serve different needs.

A citation answers many questions, often including:


Who did it? Sometimes the citing author wants to give credit to earlier work, and sometimes the reader is just curious.
Is it new to me? The reader often wants to know whether they are already familiar with the cited work. I use a combination of authors, title, publication venue and/or year, depending on the article. Other people may prefer using other information.
Where can I find it? Traditionally we needed information like publication venue, publisher, volume, issue, and page numbers. DOIs and URLs are often better in electronic publications. Many people just do a web search with the title and use the rest to confirm that the returned result is correct.
Where was it published? Some people are interested in the prestige of the publication venue. Many want to keep track of interesting venues.
When was it published? Publication years help to establish a rough timeline of events.


The in-text style also depends on the typical amount of citations in the venue/field. The less work you cite, the more of the above questions you can answer in the text, and the reader does not have to jump between the text and the bibliography. On the other hand, verbose citations can get confusing, if you have too many of them. In such cases, it is better to refer to the bibliography.