Norinaga's most important works include the Kojiki-den (Commentaries on the Kojiki), made over a period of around 35 years, and his annotations on the Tale of Genji. Using the methods of Kokugaku and Kaozheng, Norinaga claimed that the Kojiki was the oldest surviving Japanese text. He used the supposed antiquity of the Kojiki to develop an idea of indigenous Japanese religion, laws, and religion which were later used in the development of an idea of State Shinto.
本居宣長's Japanese Wikipedia page has a similar paragraph:
Apparently in his attempt to seek Japanese nativist philosophy Motoori Norinaga chose to develop a new concept as a counter to Buddhism and Chinese influence, both foreign and thus undesirable in a time Japan was pining for national identity and national ethos, whence Shintoism was later born and propagated.
But I wonder: was the concept of 物の哀れ developed for the sole purpose of Genji analysis? I haven't read Kojiki-den, but I wonder if Motoori had used the idea of mono no aware in his earlier works. Where did that idea come from? What inspired and informed his conceptualization of mono no aware?
It's understandable that he chose Genji. Again from his Wikipedia page:
Hitherto scholars of ancient literature had shown a preference for the grandness and masculinity of Man'yōshū poetry and an aversion to works like the Tale of Genji, which were regarded as unmanly and feminine. Norinaga resurrected the position of the Tale of Genji, which he regarded as an expression of mono no aware, a particular Japanese sensibility of "sorrow at evanescence" that Norinaga claimed forms the essence of Japanese literature.
But I don't see any reason he wouldn't use his newly minted idea on other classic works of Japanese literature. I am very curious about his sources of inspiration. Where did the idea of mono no aware come from?