It simply didn't make enough money.
It's easiest to see if you compare: The first Power Rangers movie grossed a total of $66,433,194.
Turbo: A Power Rangers Movie grossed $9,615,840. See how much less that is?
If you're wondering WHY it did so badly, it was due to a lot of things. First, it failed to explain where the Turbo powers came from. This was mostly because David Yost (Billy Cranston) was written out of the show shortly before. He was originally supposed to create the Turbo Powers himself, but since he was sent away to Aquitar, there wasn't any explanation given.
Second, Steve Cardenas (who played Rocky DeSantos from MMPR2 through Zeo) injured himself and retired from acting. In his place, Justin Stewart was put in. Justin Stewart, being much younger than any other Ranger to that date, alienated a large portion of watchers who were older. The character wasn't well-defined and overall was not very popular.
Finally, several scenes were cut from the movie, including character development for Justin, an alligator (or maybe crocodile) fight scene, and a few others.
Many of these reasons and more were why the Turbo movie as well as the entire season did so badly, that the Power Rangers franchise almost ended with it. The In Space season managed to save it.