That is indeed a mystery. Could be deep rooted patriarchy because even north Indian actors aren't accepted in south films except for few in negative roles & those rare friendly appearances. But the north Indian or non-south Indian girls have gone onto rule the south film industry & that list is big.
And this has been the case since late 90s. And it wasn't just actresses without market turning to south. Juhi Chawla was accepted big time in south but she chose to continue in Hindi unlike a Sonali Bendre who got bigger south films opposite big superstars after they lost their Hindi market.
Shriya Saran, Kajal Aggarwal, Tamannaah Bhatia, Genelia D'Souza, Bhumika Chawla, Rakul Preet Singh, Late Aarthi Agarwal, Aarthi Chabria, Namitha to newbies like Kiara Advani... Like I said it's a never ending list..
Akshay Kumar was not accepted when he did Vishnu Vijaya in Kannada. It was simultaneously shot in Hindi as Ashaant. Considering how it flopped in Hindi & did well in Kannada for long time Akki believed he was rejected & his co-stars legend Visnshuvardhan was the bigger superstar than him.
The likes of Suresh Oberoi to the recent addition to south Vivek Oberoi, have all tried their luck as villains after losing market in Hindi. Otherwise it's like Sonu Sood who struck gold as villain as he was ignored in Bollywood.