Here's what happened:
In 2006, Peter Jackson (director of Lord of the Rings) and Neill Blomkamp (director of District 9 and one of the Halo live-action shorts) were ready to film a Halo film, with Universal Studios and 20th Century Fox to back the project.
However, in October 2006, both Universal and Fox left the project, citing the high budget and the box office failures of recent Video Game film adaptations. Since then, Jackson and Blomkamp moved on to different film projects.
Microsoft still express interest in making a Halo film