The Wikipedia article actually covers the answer to this fairly well:
It is a compilation of sermons and other materials from the early
years of the Church, which were transcribed and then published. ...
Questions have been raised about the accuracy of some transcriptions.
Modern technology and processes were not available for verifying the
accuracy of transcriptions, and some significant mistakes have been
For perspective, see the Encyclopedia of Mormonism article on Record Keeping. Among Latter-Day Saints, anything that was not properly recorded is not considered "official", in order to prevent confusion resulting from inaccurate records.
The sermons from the Journal of Discourses were written down unofficially by people in attendance when they were given, and in the absence of modern recording technologies, they were generally transcribed while they were being delivered, in some form of shorthand. Obviously, this is not an optimal set of conditions for preserving accuracy, especially as it was common practice during those days to throw away the shorthand notes once they had been "decompressed" into longhand.
This allows for two places in which transcription errors can enter the material, with no way to verify their accuracy later. It's not surprising, then, that "some significant mistakes have been documented" in the final product.
Because of this, the works preserved in the volumes of the Journal of Discourses, as interesting and historically valuable as they may be, cannot be considered canonical by the Latter-Day Saints.