[raincom]

There are many factors: (a) Prof. Sheldon Pollock notes that there is complete silence in eight domains from 10th century to 15th century. This is due to Islamic Rule.

(b) Bhakti movement did not emerge in the south India. In fact, it is a product of North India in response to Islam. Prof. Jack Hawley made the case for its being North Indian product in his 2015 book.

(c) Prof. SN Balagangadhara in a talk provides another evidence: Bhagavadgita itself notes that gyaana maarga (the research tradition) died out even before the gita time. Upanishads and Brahma sutras were product of this group of researchers (or gyaana maarga).