The sidewaystable environment does not let you split its contents across multiple pages. Since you're using a longtable environment, which has its own \caption commands, etc., there's no real need to embed it in a sidewaystable environment anyway. Instead, consider loading the pdflscape package and using that package's landscape environment to rotate the material by 90 degrees.

Addendum to address a comment by @pauljohn32: If the longtable and pdflscape packages are loaded and if a halfway-competent pdf viewer is in use, the landscape/longtable combination (a) rotates the tabular material 90 degrees counterclockwise to landscape format and (b) ensures that the onscreen display of the pages with the tabular material is also rotated, so that readers don't have to crane their necks in order to take in the tabular information. However, this combination does not make the pages into "landscape pages"; i.e., the header and footer lines are still placed where they'd go in portrait mode. In the following screenshot, I've highlighted the MWE's header and footer lines in yellow to demonstrate this fact. (Of course, what exactly is placed on the header and footer lines depends on factors such as the document class in use. However, that's a separate issue, isn't it?)