Yukata was originally said that it was a unlined kimono worn by a noble lady when she bathes. Then it was widespread when people started wearing colorful Yukata when they joined bon-odori (Japanese traditional dancing) events which became popular in the beginning of edo era.
Because of this comfortable style, it became popular as a "casual kimono" and even now it is used as indoor wear at Japanese-Inn of Onsen (hot spring).
In modern times when Japanese people became to wear a western style clothing, there are many people who dress up with Yukata to go to summer festival, bon-dancing or fireworks festival. In addition, it's easier to wear Yukata than regular kimono, so it started to catch attention of young generations again.