Luckily for, well, both of us, AltJapan has the fully-translated liner notes that explain the history of the production. The short version? Toei's projects were finishing up, so they started work on a deal with Marvel to make TV shows based on its popular heroes. The project was intended to usher in a new generation of heroes-- heroes that didn't “transform” --and altered several aspects of the Spider-Man story. For example, his powers came from “Planet Spider,” not a radioactive spider, and his enemies were less an assortment of colorful wackos and more an army called the “Iron Cross Army.”
The long version, of course, is right here. Read it!