Because all of those sites are established and have unparalleled content. There are no competitors to those websites.
If they are the only game in town, then I will put up with bad functional design and if needed crawl on my knees to solve a problem of mine.
I remember years back trawling through multiple expertsexchange pages to solve some coding issues.. Now I don't as better options have come along...
Why is the same not happening with craigslist, cause they do the basic job well.. have a functional designed site which solves peoples problem easily.