Or, people that thought it is too big for one part will sell it in multiple parts. Guns 'n Roses best albums are Use Your IIIlussions 1 and 2, and it is sold in two parts because the content does not fit one CD/Cassette.
Fellowship Of The Ring, The Two Towers, Return of the King. Three separated books that with all the pages added, still have less pages than some Stephen King book. Still one of the best written stories.
Kill Bill Vol1 and Vol 2. Even if is intended as one movie, it was released as two. (https://www.imdb.com/title/tt6019206/)
Final Fantasy VII. Released as one game. One complete story.