(edit) I think every downvote should be public with their name and reason behind it.
Something Steam has been doing is breaking down why a particular game is recommended (reviews, popularity friends, interests, other metrics,..) why not expose this data to users for news?
We build these systems to filter out the noise for us, but that doesn't mean we should be beholden to a black-box deciding what content we should see. The filter should be ours to tweak and knowing how it works is a first step to that.
Quite simply, because Steam is not Google. Steam's value is in all the games you want to play being on Steam, not in its discovery algorithm. It loses nothing by exposing that algorithm because even if you had it, you would not be able to build Steam as you do not have the games that are on Steam. It has vendor lock in.
There's no such lock in for Google. Google doesn't have exclusive rights to the websites it indexes and anyone could, it they had an algorithm as good as Google's (and the computing resources) build Google.
The only tech factor holding Google in place as the #1 search engine is that its algorithm is better than everyone else's. The algorithm is intellectual property that Google has invested billions of dollars in developing.
Providing the weightings (and allowing as some people are suggesting the ability to hobble Google's algorithm by using your own) would be the first step towards someone reverse engineering Google and be a material business risk for them.