It is a part of the news biscuit culture that we applaud good work, but do not tend to criticise (constructive or otherwise) weaker pieces. I was lucky enough to hit tickers, Nibs and an FP very early in my NB career but it is difficult for somebody who isn't writing in the target style to be informed of their mistakes.
On my part the lack of feedback comes from 2 reasons
1) A lack of time, most of my posts are written during my lunch break and not when I have access to NB. They are then cut, pasted and given a brush up on the site. I therefore don't spend ,as much time looking at other peoples posts as I probably should.
2) Given the amount of final edit my work goes through to get on the main site. I doubt that my feedback beyond the bleeding obvious (i.e. add punctuation, add jokes and get the story to a usable length) will make for a story that is more usable by the editors.
I am willing to spend some time once a week in searching through the nearly stories and new writers pieces, giving constructive feedback, but it will feel horribly like work and that's not why I write for this site.