Hi All,
We have been tracking this issue since it was originally reported, many months ago. Based on our investigations, we believe the issue to be on Google’s end, not ours. See this thread on the Google Product Forum.
As a potential workaround, our tests have shown that the Drive app suggests SketchUp Viewer as an Open In option only if the SKP file is uploaded to Drive using the Drive desktop app. When files are uploaded to Drive via a web browser, the Open In function fails to suggest SketchUp Viewer as an app to open SKP files. 
We’re monitoring the status of this issue and will report back if/when we hear some good news.
In the mean time, sincere apologies for the inconvenience.