Unless you're a big big Power Rangers fan, you probably didn't know this one.
The Power Rangers are actually a Japanese import from a long running series called Super Sentai. The basic premise is the same: a team of (almost always) 5 people are chosen to magically transform into super warriors to fight evil.
Have you ever noticed that the fight scenes when the rangers transformed, and all the Zord scenes always looked completely different than when you were able to see the rangers' faces? That's because the show basically reused footage from the Japanese show.
If you're reading this, the Power Rangers probably defined your childhood in the 90s. You'll be surprised to learn that Super Sentai series has actually been around 1975. In fact, that recycled footage I just talked about? That was from the SIXTEENTH season of the Super Sentai series.
Yes, there were 16th seasons of Japanese Power Rangers before they became famous in America.
When the first season of Mighty Morphin' Power Rangers was a success, the American producers asked the Japanese production company to please shoot more fighting scenes so they could make more episodes of the MMPR show. However, that was all they did.
After that, the Western production simply rebooted the show every time a new Japanese series started. That's why the early Power Rangers changed costumes so often and that's also why nowadays, they just start a new series every year.