I thought that you were supposed to use the Oxford comma, but how come people don't use it anymore?
It is still recommended by the majority of American style guides. Some people think the Oxford comma is redundant, since you are already saying "and," which is kind of the same thing as a comma in itself.
I would say a good rule of thumb is to use it, particularly when it helps you avoid ambiguity. One example of a sentence that benefits from using the Oxford comma is "I want to thank my parents, Eric, and Molly." Let's say Eric and Molly are your siblings. If you take out the Oxford comma it will seem like they are your parents: "I want to thank my parents, Eric and Molly."