- ARBITOR 5
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- Exalted Mythic Member
Well, I think its because they wanted to have more Dual Wieldable weapons:
Halo Wiki:
Compared to Halo 2's default starting weapon, a single SMG, the Assault Rifle is far more effective, but only single-wieldable. This reflects Bungie's desire to move away from the "dual-wield rush" that characterized the beginning of a Halo 2 game. The Assault Rifle is powerful enough on its own to fend off a stronger enemy, descope a sniper, or serve as a medium-range weapon. The Assault Rifle can be effective at ranges even further than "Mid". The SMG has also reappeared in Halo 3, but is considerably more effective than Halo 2's version.
See here for more.