You never needed those two probably. Because Itachi was the first character in the story to display Susano'o , Amateratsu and Tsukiyomi, and he stated he awakened Susano'o with those two powers , the analysis stuck with the only info they could go on. In order to not give away info on other potential Susano'o users jutsu, it stayed with Itachi's description of the jutsu.
Its not that its only Kishi's writing at that time, but its more so the characters data or notes . Because Itachi needed them for Susano'o doesn't mean everyone else did.
Kakashi being given Susano'o and two MS with Obito's power was due to Obito mimicking Hagoromo and Ashura/Indra. Transmigration. He was a juubi's jinchuriki and Tobirama also claimed to possibly use his ghost jutsu the same way Dan used it to replenish Tsunade's chakra , but by doing it to Sasuke. He *obito* also has proven to skip steps when it comes to his sharingan powers.