3. Those behind large media conglomerates all happen to have the same rules, incentives and culture. Certain stories are collectively suppressed not just because "they wouldn't be interesting to US public" but because it would undermine certain assumptions and values that are held in the large corporate culture.

Read "Manufacturing Consent" book for a more thorough explanation on how the system works.

(It works on individual level -- journalists and corporate --- certain stories are suppressed by the owners).