The author supposes that there are some mustache-twirling masterminds behind the scenes at Google, Twitter, and Facebook, picking and choosing what content is promoted. But the algorithms, at their most basic sense, promote what is popular.
Based on voting records, in absolute numbers, there are simply more left-leaning people (at least in the US) than there are right-leaning people. Gerrymandering and the electoral college, etc don't reflect this in the government, but it is the case.
The left-leaning viewpoints are more promoted then, simply because there are more people who want to see them. There's no reason to posit a nefarious conspiracy.
We could argue that it would be beneficial to promote less popular viewpoints more to expose people to new ideas. It probably would be a good thing. But these companies are in the business of making money and people generally just want to see things that confirm their own biases. Just look at how upset conservatives are that they can't find enough content like that ;)