My first website was hosted on AOL Hometown (also gone), primarily because it had more disk space than Geocities, unlimited bandwidth, and no hotlink protection on my files. (and for awhile, it was ad free)
I had a site that supplied backgrounds, clip art, and midi hotlink codes, for use on sites like Blogger, Live Journal, Dead Journal and Neopets, which didn't allow uploading files. This was back in the days before there were any free image hosting sites like Imageshack and Photobucket, and people had a lot of trouble finding places to host their files for use on other sites, for free.
I see that Reocities went only for the sites that had the traditional area + number urls. There was a point where Yahoo stopped doing that and site url's were based solely on username, without the area designation.
I guess all of those sites are not included in this archiving effort and it was just limited to the older ones.
Another thing that might have made it more difficult to archive these sites was that Yahoo instituted a 4mb/hour bandwidth limit. If you tried to archive a site bigger than 4mb all at once, you would end up knocking the site out for everyone and having to wait an hour to obtain more of the files. (I ran into this issue when trying to archive a friend's 10mb Geocities site)
This bandwidth limit was common knowledge and was often used to DoS attack sites by downloading them repeatedly till they hit the bandwidth limit. It was also why you couldn't submit any Geocities URL's to Digg.