Why are some games not available to download?
A game tagged "NOT abandonware" are, simply put, games still on the market hence "not abandoned".
Abandonware is a strange definition. Abandoned games are not really free; the original developers (or publishers) ceased to exist or doesn't care anymore. Some old games were never released for free or liberated and some of the oldest software companies are still in business and prefer to protect their intellectual property.
I try to pay attention to online publishers in order to avoid uploading games still sold somewhere in a way or the other. It happened more than once that someone kindly wrote to me asking to remove a game from AbandonwareDOS and I always promptly complied.