Speaking to anonymous sources privy to the development process, Thurrott's report notes that the delay was caused by the game being developed on a new engine while also targeting new hardware.
Halo Infinite Delay Explained
Imagined as a spiritual reboot of Xbox's most famous franchise, the report notes that despite being in development for a few years, the game's staff shifted multiple times.
According to the report, much of the development was outsourced to third-party studios which caused frictions among key staff on the project.
Creative director Tim Longo left 343 in August 2019, while his successor Mary Olson left just months later.
The full report is well worth a read, noting the disparity between the game's E3 2019 showing in comparison to the big gameplay reveal in July 2020.