Ok, so here is why things happened the way they did. Mighty Morphin' Power Rangers was based on a show called Zyuranger in Japan, and reused footage from said show. In this show, the Green Zyuranger dies during the course of the series. Originally Tommy was written to have been supposed to die in the American show as well, but they saw he was a fan favorite and kept him in using stock footage here and there. Eventually they wrote him off as they ran out of footage and said his powers ran out.

Fast forward to the next year, in Japan there is another series known as Dairanger. This show was the basis for the Power Rangers second set of Zords, but more importantly, they did not have a green sixth ranger, but a white one, with a Tiger Zord. The white ranger was portrayed as a child who magically transformed into an adult state when he was in his ranger form. This is why Tommy acted more whimsical and funny during the second season of the show, because the footage showed a white ranger jumping for joy and being overall ridiculous.

The next year was called Kakuranger, and their robots and monsters were recycled into the ninja zords and new monsters for our show. The rangers themselves appeared as the Alien Rangers of Aquitar. The year after that, Tommy became the Red Zeo Ranger in a series based off of Ohranger in Japan. I could keep going but I don't feel like it, thats just a brief history :)