It seems to be in large part due to (1) articles that require strong background knowledge tend to only be edited by appropriately knowledgeable authorities (e.g. higher level maths, quantum dynamics) and (2) editing privileges for highly controversial articles tends to be very tightly controlled.

There's also an interesting trend wherein Wikipedia editors tend to drift to the political center: https://www.washingtonpost.com/news/wonk/wp/2016/10/25/somet...