If you want a passable technical reason for a Pelican and not a Falcon or Hornet, then I'd say altitude.
Both the designs of the Hornet and Falcon (which isn't a true tilt-rotor) would limit them and possibly make them impossible to be used on that level.
Not that I'd assume anyone at 343i was thinking along those lines though.

The H4 Pelican took most of it's cues from the Mi-24D and beyond, with it's tandom double bubble canopy and no connection between cockpits and troop bay.
An evolution of design as it's moved away from armed troop carrier to gunship capable of carrying some troops.

And right now, there is this beast in the air.