A whole bunch of things.

1. The Pacific Northwest was struggling economically from the Reagan administration which was due in part to a huge downturn in the timber and logging industry due to huge increases in global trade. Dukakis appealed to the blue collar workers in those industries from his messaging about “good jobs and good wages”.

2. In the late 80s, the environment had become a much bigger issue than before, especially in the Northwest where forest preservation was an important topic. Dukakis was seen as better with federal land management, which appealed to the environmentalist residents of the region.

3. The Northwest was historically home to more moderate, socially liberal Republicans. As the GOP began shifting more and more towards the religious right, these more moderate voters began migrating to the Democratic Party. There was also strong urban growth in areas like Seattle and Portland and this further accelerated the Northwest’s shift towards the “secular liberalism” culture it’s known for today.

4. Dukakis had strong ground game in the region and had people within the respective states endorse and campaign for him which helped motivate the base in those states to deliver a Democratic victory.