There were lots of reasons: no unlimited weapon caches, skulls which permitted specific gameplay (the one to melee to acquire back your shields, awful), tiny jump height, slow base speed, and the stamina system, which made health packs and their importance that much greater.
You were a more powerful player in Reach, thus pushing some of those settings out the window.
They made changes in remakes (Hemorrhage base design notwithstanding), and they were far better for gameplay. Beaver Creek's dirt ramps, Warlock's air lifts and bottom center to spawn bomb, Coagulation's base design and general terrain/TP area), and the subtle changes in Desolation and Tombstone were pretty sufficient. The ledge by the Needler and the underneath hallway by red team OS in Tombstone, swapping team bases so the larger base is defended, rather than the attacker, for example.
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