Technical reports and journal articles are both used to report the results of research and development projects. There are differences between the two that are driven by the objectives of each form of reporting.
The primary objective of journal articles is to report results of experimental and/or theoretical scientific investigations to enhance the body of scientific knowledge. This is the primary way that (1) science advances and (2) the scientific community communicates among its members and practitioners. Typically, there are space limitations prescribed by the journal publisher that limit the length of journal articles usually to only a few pages. Journal articles are almost always subjected to a rigorous peer review process before they are accepted for publication.