Why would you think the mainstream media isn't covering it? A cursory search on Google for 'oregon drug legalization' revealed these results:

https://abcnews.go.com/Politics/wireStory/oregon-1st-state-d...

https://www.washingtonpost.com/nation/2020/11/04/election-dr...

https://www.usnews.com/news/best-states/articles/2020-12-10/...

I would consider these outlets more or less mainstream. I think the 'real story' in this case is not a conspiracy of big pharma and the prison-industrial complex, but the slow change of attitudes and policy in the US. Occam's razor.