Why do some letters get accepted, others rejected?

I second Nigel Pike’s arguments concerning the protective shield obviously put in place by The Nation’s editors. I myself consider the 50-per-cent “failure rate” for publication of my letters – usually dealing with Thai education – acceptable. 
I analysed the possible reasons for why my letters might be parried by the editor’s shield and came up with: lengthy “emotion-driven” content. I suspect that even well-meant, constructive letters cross a red line if they stray into the territory of rant, emotional plea or heartfelt “essay”.     
Even when driven by honourable intentions, knowledge, insight and righteous anger, our thoughts must be expressed wisely, as Nigel says. Lack of care taken here can have irreversible consequences, and it’s not always necessary to flaunt our knowledge or become embroiled in argument whose only object is to seek agreement or disagreement with our peers. 
Dirk Sumter