Future Halo Titles to Use Unreal Engine
halowaypoint.com
halowaypoint.com
Back when Halo 1 was launched, it was one of the first games to use dynamic octree subdivision on height maps combined with indoor portals, which meant it was superior to the Unreal Engine w.r.t. large landscapes with caves at the time. By now, of course, all other game engines can render similar landscapes. And that's why the original USP of the Halo engine is now moot.
I imagine the real reason is that using a standard tool:
1) Hiring, it is really hard to hire people who can do core game dev engine even if you don't need a lot of them 2) Less synchronization required between teams 3) Less work being blocked by system programmers or lack of good tools 4) Lower churn of people leaving because of bad tools