I actually think it has to do with the fact most people don't think old computers are useful. A 15-year-old TV is still "good", all it has to do is display a decent image and people are happy. What would anyone do with a 15-year-old computer though? We may have the answers but many people have no clue. So it takes specifically requesting these old computers before people realize there is someone out there who does have a use.
Our local freecycle has a better variety but electronic things are almost entirely TVs or appliances (sometimes a computer monitor), almost no computers of any sort. And plenty of baby/kid junk of course. I haven't had the room or gas money till recently to try putting out a request but will give it a shot soon.