It is obviously a commercial reason. Although the filmmakers argue with you and say otherwise. I was in Ralph Nwadike's office doing an interview and I asked him why Survivors was in more than One Part. He said after dropping several scenes, he still had more than 3 hours of footage - and that cannot fit into one part. Is it that our screenwriters don't know how to guage what will fit into 90mns? Or they want to pay Genevieve for one movie, while getting two out of it? Those who know better have suggested it is the sign of lazy writing for a story to go more than one part. Use your scissors. Cut the thing. Too much dialogue? Many scenes can be deleted, especially in the yoruba movies where they plug in Papiluwe or Baba Suwe for that totally irrelevant comedy aside. You watch the bit and you wonder, what the heck has that got to do with the movie? Some stories will require that you go beyond 90mins of course, but that should be part of the idea from the start, and the screenwriter should have enough material to fill up the time, not adding dialogues to pad.