Obviously because you pick the races in which you have the best chance of winning. Libertarians are conservative and have a conservative message. Conservative messages obviously don't do well in seats which are already held by Democrats. This is especially true when it comes to things like tax increases or tax cuts.
In my home state of Indiana, the LP fields a candidate for every election. In Oregon, I'm guessing the LP's funds are slightly more limited, and so it picks and chooses the seats at which it thinks it has the best chance of winning.
Actually the official LP position is very socially liberal. Maybe that is why they don't target democrats.