Why are there no new chapters of Naruto Gaiden?
Naruto Gaiden: The Seventh Hokage and the Scarlet Spring is a sequel mini-series of the original Naruto manga, part of the Naruto Project. The Naruto Project is a special project to celebrate 15 years of the Naruto series.
The Naruto Gaiden acts as a prelude for Boruto: Naruto the Movie.
In Japanese literature, a gaiden (外伝) means side story or tale.

The use of gaiden is commonly used in popular Japanese fiction to refer to a spin-off of a previously published work that is neither officially considered a sequel nor a prequel. However, some gaiden are retold stories in the perspective of a different character, similar to that of a flashback.
Wikipedia

Therefore gaiden is generally consists of only a few chapters.
The unexplained elements in the series might (most probably will) be elucidated in the upcoming series.