I think it is combination of two factors. 1. A subset of HN readers are uninterested in items outside a narrow range of topics and wish to keep HN focused inst

I think it is combination of two factors. 1. A subset of HN readers are uninterested in items outside a narrow range of topics and wish to keep HN focused instead of becoming like other forums that grew to cater to broader audiences. 2. Forum moderators may have a financial incentive to ensure that certain topics are discussed more often and other topics are discussed less often.