During a recent interview with IGN, 343 Industries boss Bonnie Ross revealed Microsoft seriously considered the idea of Gearbox becoming the new developer for the Halo series after Bungie left.
At the time, Microsoft was looking for a new developer to handle the series and Gearbox was their top choice. Main reason was due to Gearbox handled the PC port of Halo and had the most experience with the series after Bungie.
In the end, Microsoft decided to form an internal studio to handle the series, which led to the creation of 343 Industries.
https://www.ign.com/articles/2019/02...ign-unfiltered