There are loads of old sites no longer around. Some disappear because life situations change and those running them can no longer do so. Some get shut down when hosting services change their policies...or shut down themselves. There are probably as many reasons for a site disappearing as there are sites that have done so.
My site has a listing of many old closed sites...most of which have at least partial remnants left online. If you go HERE, you'll find some that I've resurrected and a link to the complete listing of those I looked up on the Internet Archive Way Back Machine. It's not the same as having them still around and active. But, at least it's something. 🙂