There were broken necks, slit throats and severe trauma with blunt objects but should that be able to kill vamps and lycans? I thought it was silver, sunlight, and decapitation.
And what the budget allows. Having most of the people fight with guns and crossbows, getting shot, and then simply dropping dead is cheaper than having numerous CGI tranformation sequences and scenes of the two sides battling it out in their non-human forms (flying through the air, throwing each other around, ripping and slashing each other to shreds, etc). Although THAT would have been way cooler.
When I'm good, I'm GOOD. When I'm bad, I'm BETTER.