I would say for most it isn't a question of being lazy or ungrateful, but simply a factor of not knowing what to say or not having time. I myself don't post much because I don't have anything viable to offer beyond the variations of "good job" - which I'm sure are not unappreciated but it tends to strike me as forced and dishonest when I come up with the same old reply every time. If what I post doesn't sound genuine to me and is only there "just because", then I don't want to insult a fellow modeler's work by putting in filler comments. But again, that's my viewpoint.
Someone earlier in this thread made a remark about how there are modelers who only have their own site and "little homes" they live in and apparently don't consider the rest of "us" worth the trouble. I have to take exception to such a remark, especially looking at how the bulk of modders in any community seldom chime in graciously at every piece of work posted by their peers. 99% of the posts in this very forum are about showing off what someone has made, so I would say such remarks are poorly considered.
Many of the regular posters here are modelers and the bulk of replies to new ship releases are by fellow modelers. For me, I take that in stride as criticisms from another hold more weight when they are familiar with the process. Not to say non-modder comments and suggestions aren't acceptable, but it is a natural human tendency to let a peer's words take precedence over anyone else.
Some of the reasons stated earlier, such as the age of the SFC and the decline in popularity of these sorts of games in general are valid. However, I have to point out that we haven't helped this situation much either. Not to go off on an "old man, had to walk 10 miles in the snow to school" speech, but I remember the ego-fests and flame wars and flinging of...stuff...across boundary lines so many years ago. Mindlessly competing with each other, spreading lies, arguing and debating and thieving and worrying - all for no other reason than our own delusions of grandeur. Factor in the times our work was blatantly ripped off or stolen or our sites subverted by community leeches and unproductive moochers, and you have plenty of fuel for forum troubles.
Hell, there have been times when other posters went off on each other for daring to critique "the masterpieces"...running off fresh faces to the forum and giving everyone - including the modders - a bad reputation. Why would anyone want to post any comment to a thread after seeing modelers treat each other in such ways, let alone the regular players? Thankfully such attitudes have leveled off and I'm glad to see that some of the more self-centered individuals are no longer among us. Unfortunately, that perception of us being egotistical, short tempered prima-donas is still with us to the point where if we even try to defend ourselves from legitimate wrongs, we get accused of whining or "not supporting the community".
I would bet that many of us, after all these sorts of mixed messages and troublesome times, would retreat back to the barest minimum of community interaction and mind our own frakkin' business, as it were. Not that I wish to imply any personal experiences in that statement or anything...
I personally stopped getting upset over the lack of replies a long time ago; it certainly has no bearing on how good or bad the work is. I learned to quit bothering with prestige and glory and to just do things my own way on my own terms, so if anyone is here for attention then I suggest you stop right now. I'm here for the hobby and the distraction and to give back to the community in my own way. SFC is the only community worth a damn as far as I'm concerned and if it weren't for what I've learned here, doing whatever it is I do, then I probably wouldn't be where I am now. How many people can say they got a job with just a hobby website for a resume?
Anyway, to steer things back to the topic - I doubt we can beg or coax people to start posting more, as we've all been trying that since before Dnet even existed. It didn't work then, and it certainly won't work now. The best thing any of us can do is to simply make what we want, why we want, when we want and let things go on naturally. And if, by some chance, we do get more than a few token replies, well that can't possibly be a bad thing now can it?