Q. Why isn't wikpedia a credible source?
Because there is limited editorial control, since anyone can post information, and their sources and credentials aren't verified, as with a book or journal. Actually, in recent years, additional editorial controls have been built into the Wikipedia product. You should also keep in mind that the internal links that you find within a Wikipedia entry can also be fruitful sources of information and should be evaluated independently, on their own merits. This is also the case with the lengthy bibliography of sources, attached to most Wikipedia entries, many of which are from reputable scholarly journals.