Why are there no familiar regional film industries in north India in contrast to the south?

There is a fairly decent sized film industry in Bengal. Satyajit Ray was a towering figure there, and he had many contemporaries. But here too, you find artists migrating to Bombay in search of better chances. Bollywood is full of actors, directors, music directors, writers, and other back-office functionaries who have migrated from there and working for different regional industries.

If you look at census figures, the number of speakers of Tamil (72m), Telugu (74m), Kannada (55m), and Malayalam (33m) do provide a small but significant captive market. Bengali has 83m speakers. While Gujarati (60m) and Marathi (112m) do have large numbers, these were a toss-up between being considered a dialect of Hindi or a language. There is indeed a small Marathi film industry.