The world's largest snakes are generally considered to be located in three major areas of the world: Indonesia, the Amazon, and the western United States. If you are talking about the Pacific Northwest, in comparison to Indonesia and the Amazon, the factor that all three have in common is WATER, and lots of it!
Apparently the presense of moisture is important to the growth of a snake, and the more there is of it, the larger the snake gets! While there ARE large snakes in the southwestern United States (Western Diamondback rattler--up to 8' in length), I will have to agree that the snakes in Washington and Oregon get huge!
The average annual rainfall on the Olympic Peninsula of Washington, namely the temperate rain forests of the Olympic National Park, is 150 inches, making it the second wettest spot in the United States! Therefore, much vegetation grows, and this factor alone could explain the presence of large snakes. Not only would there be sheltered places for the snakes to dwell, but the dense vegetation would provide homes for the smaller animals upon which the snakes subsist.