Why did the Japanese invent Manga? Are there any good scholarly resources on this underlooked topic?
It's not really that underlooked. Fredrick L. Schodt's 'Manga, Manga Manga' and 'dreamland japan' are probably the two big works, but there's quite a few tomes published in the past 20 years or so.
The origins of Manga - Emakimono - narrative scrolls from the 11th century, woodblock prints etc... first manga mag published in late 1800's, revival of 'street comics'
Kamishibai along with US influences during the occupation had a huge effect on Osamu Tezuka (the god of manga), who popularised a new cinematic style of comics form the 50's drawing heavily on Disney aesthetics and defining the 'shojo' (girls comics) and 'shonen' (boys comics) with Princess Knight and Atom boy.
There's a million other factors that led to the development of the media. The question to ask is 'why did the medium of comics become so popular and accepted in Japan?'