Although I agree with Cape Code that this is not a question related to academia and should be closed, I know this one of the top of my head so i'll save you some Google-ing.
The reason is because the first mass production of "paper"s of a standard size where printed for distributing the news. The word "news" is a very funny English word, for all sorts of reasons. It ends in an "s" but is not plural, eg "what are the scores?" is correct, but "what are the news?" is not. Furthermore, it is uncountable, so although it can be followed by a singular verb, you cannot count it like "Do you have a news for me?". People instinctively know "Do you have news for me?" sounds unusual, and so they often pop in an "any" into the middle to make it sound more grammatically compliant.
For this reason, early usage of "news paper" referred to a collection of 1 or more printed news. "news papers" referred to several of these collections. This eventually became 1 word, newspaper, which as a single noun now plays by the rules a bit better, masking the weirdness of "news". However, the concept of a "paper" being one or more sheets of printed information stuck, and is still used in academia and elsewhere where some form of news is being printed, but not as a newspaper.
For the record, I know all this because I got into an argument with a colleague about it, and I insisted it must be because of paper (the material) is what people are referring to. "I brought the paper!", since the common person would have only seen paper in relation to multiple sheets of it. I was wrong.