There can be a number of reasons for that.
First, not everyone specifically asks for C&C on a photograph. If comment and critique are not explicitly asked for then I don't automatically assume that the photograph was posted for that reason, I assume it was posted simply to share a shot someone felt was worth sharing.
Second, it takes time. A lot of people post photographs requesting C&C each and every day. I have a life, I have a job, most everyone here is in the same boat. If I spent 10 minutes a day commenting on every photograph that requested C&C I'd have no time left for anything else. Most of us do the best we can. If it isn't good enough I'm sorry.
Third, some photographs are just so bad that there is really no place to start. Likewise, some photographs are so good that there is really little to be said against them.
And then there are those who can be argumentative when their expertise, at least in their own mind, doesn't receive rave reviews. I just pass them by and keep going.
+1
Agree with everything here. The one thing I want to add is that the worst offenders in the argumentative department are usually not the newbies, imo. No, they are the ones who have pretty much mastered a few parts of the photo equation and are now, in their own minds, masters of them all. Very funny. And sad too considering that after more than 30 years in that business, I still don't think I don't need to learn every day. Believe it or not, I learn something useful just about every time I log in.
Now, that was what I wanted to add. What I would have changed is this: " If it isn't good enough I'm sorry." Since I am not as polite as SCraig, I woud have said: If it isn't good enough fook you. I'm not your $0 teacher.
I'm sorry if sarah ashley doesn't understand this but a number of C&C threads are showing photos of such poor quality and total lack of thinking put into them that they are an insult of sorts. If the authors of these threads were to learn some very basic things about photo before posting, they probably wouldn't post snapshots.
As mentioned, C&C takes time and I see no reason to waste mine on people who can't take theirs to learn the very basics on their own.