This is for many reasons. One of the most important is that most web browsers support the format directly without having to rely on plug-ins or helper applications. Secondly, the size of these animations is relatively small. Unlike .avis or .movs or .mpegs, the GIF89a format is a simple format and displays in a small number of colors. This reduces the size of the file.
Size is important for downloading and storage purposes.
Thirdly, there are plenty of programs that allow you to develop these sorts of GIFs. It allows you to develop your own animated GIFs as well as create variations on common ones. Some Drawbacks of Animated GIFs One of the main drawbacks of animated GIFs is the lack of color available, these files only display in 256 colors and can only appear cartoonish. They aren't a good way to do animation using photographs like time lapse series animations. Another drawback is the way they are continually loaded from the browser's cache if you are running them in a web page.