Why is the diurnal temperature range on the Cascade volcanoes so small?
I think the answer to this question can be split into two parts. First, it is important to know which weather model you are using. Weather models use grids to predict the weather. The finer the grid, the better the topography (mountains) are represented by the weather model. To get a reasonable representation of the mountains that you mention, you would need a horizontal grid spacing of a few kilometers at most.

For the second part of the answer, we will assume that the weather model you are looking at has a fine grid. The only effect that I can think of explaining the strong difference in the daily cycle between Seattle and the mountains, is the wind. At higher altitudes, radiative cooling cannot effectively bring down the temperatures since the air is continuously being mixed with air from the free atmosphere which has a much less pronounced daily cycle.