Because the evidence, at state and federal levels, shows that Republicans are tax-raising, deficit-increasing, Big Stupid Government-growing weasels.
It's not just in Oregon either. Way too many RINOs at the state levels.
I'd like to see more libertarians win office, if only just to shake up the status quo, but that won't happen. The Republicans and democrats will fight back. They have too much of an interest in maintaining an unholy two party system.