I think I read somewhere that it’s because they started to have it difficult understanding the meaning. When I read the Ghandhara Commentaries I feel that there a came a point easier made Dhamma had to be made that looked coming from the authority of Buddha. It seems commentaries was concentrated before on the old stuff. For the new suttas they probably didn’t have to make commentaries because the Sangha at that time studied the new stuff. In modern times itself you mainly understand mostly nikayas suttas. But older stuff is deep. Or you recognize a old sutta teaching by it’s difficulty. Which in formulated suttas is rare. I only find the poems at the end of suttas sometimes deep. Which probably was from old texts
I feel after Abhidhamma is still the result of Dhamma having been lost in India. Mainly because not many was getting good result as was written in old and new suttas. We can see study became more important than practice.
It’s interesting that we found a commentary on Samgitisutra because it kinda is proof that it might have been one of the only texts the previous generations had together with texts like the earliest part of suttanipata. For me commentary is a indication of something that needed to be explained because most don’t understand this old text
A Gāndhārī Commentary on the Saṃgītisūtra