1. the line delivary was better
2. the voice was more intimidating, fitting a feared bounty hunter
It's really easy to explain too: a voice modulator built into the helmet to make himself more frightening, to build on his reputation and such.
Remember when Leia was disguised as a bounty hunter in Return of the Jedi? To say that her voice then was very different from her own nautural voice is an understatement.