I gave this answer to a very similar question before and went looking for it, only to learn that my answer was collapsed. I have no idea why. So maybe this one will also be collapsed, but I think it’s a good answer. Here’s my take on why balloons became a big deal for events.
Balloons have existed in various forms for a very long time. The first rubber balloons appeared in the early 1800s, and mention of other types of balloons were found in literature before that. I’m not sure what metric we should use to determine the point that they became popular for decor, so I’m going to pick the late 1980s. It was at that time that there was a critical mass of professionals using balloons that the first professional balloon art conventions started appearing. So clearly there was demand for it. And of course, once you had organizations and conventions forming around balloon art, the popularity could grow at a much more rapid rate.