Vyk, what leads you to believe that the decision to have Hawke as a human only is not a story based decision? The story is the rise of the Champion of Kirkwall who escaped from Lothering (which is important insofar as the resulting encounter with Flemeth). I don't know about you, but I don't remember many races other than humans hanging around Lothering (there was the elf family and Sten, but that's about it).

Beyond that, voices do matter. Humans are intentionally given Received Pronunciation English accents (most are hardly perfect, but that was Bioware's stated intent) while City Elves and Dwarves (although they originally wanted to use German) are intentionally given a Midwest American accent and the Dalish are Welsh. Beyond that, merely giving a character a voice enforces the necessity for an identity. Quite frankly, if my dwarf character spoke the same way as my elf character, it'd drive me insane, and it's not fiscally sound for Bioware to record every line in the game six times.

For reference, for Antivans they hire Spanish voice actors and French for Orlesians. As I'm sure everyone already figured out.