Rights holders change because companies go under, are sold, or liquidate their library (the movies and stuff they own or have rights to). The library is purchased by somebody else. Easy enough. But when producers, directors, actors, etc. and other studios who have partial ownership or distribution rights of the movie, things can get complicated during that library transfer.

Further complications arise when people die or they sell the rights to another person or company. Then you can have this knotted mess of home video and theater distribution rights (those can be separate and owned by different entities too!) which can culminate in lengthy lawsuits, or disinterest, conflict over money and percentages of profit, original contracts with the director/producer/actors/etc. getting in the way somehow, misplacing or destruction of film negatives (by far the worst case scenario), or straight up people not even knowing they own a movie's rights.

Then there's the stuff other commenters said. I missed some stuff I'm sure, and am not an insider. Just love movie news and movie production.