There are a great many reasons why software might become abandoned. The useful life of hardware and software is often only a few years. As old hardware and operating systems are replaced with newer and more powerful versions, and new applications are created to take advantage of the improved capabilities (like faster processors, more memory and better graphics), older software eventually becomes obsolete and falls out of use. And after a system upgrade or two, a lot of software becomes incompatible with newer computers and gaming systems, especially if the companies don't work in backward compatibility. To make matters worse, the hardware and software media eventually degrade and stop working.
Game studios and other software producers are also prone to going out of business or getting sold to other companies, so the rights to game titles change hands frequently. Some companies may not even realize they own a particular title, especially if they weren't the originators of the work and it's been off the market for a while.
Another possible reason for abandonment of software is a company deciding that it is no longer commercially viable to spend time and money on advertising, distributing and supporting an old game, or porting it to newer platforms. Heck, even new widely anticipated games sometimes get killed before release to save the studios money.
A copyright owner might intentionally make an older game unavailable to encourage purchase of a sequel (although companies have been known to release older versions as marketing for a sequel, as well). Software might also only be temporarily abandoned, either put out of print to release again later or abandoned for a while before being recreated for a newer operating system or console.
The concept of an orphaned work is similar, although not all abandonware is orphaned. An orphaned work is one where the copyright holder is not known or cannot be located. Some abandonware is orphaned in this manner, but for a lot of it, we know who owns it, but they're no longer distributing or supporting it.
And occasionally, a formerly abandoned software is voluntarily put into the public domain or made publicly accessible by the owner, making it freeware rather than abandonware.