It wasn't. "Arizona" is merely the name given to its location by settlers!
If you mean why did it form there as it has, then that's a happy co-incidence of geology, geomorphology and precipitation over many millions of years.
It is consistently where the Colorado River is or where it has been. The river has a degree of mobility that over eons has alloowed it to pick, choose and meander at will.
True.
It is in the northwest part of Arizona.
The Colorado River.
The Colorado River.
The Colorado River.
The Grand Canyon is in Arizona and was carved by the Colorad River.
The Colorado River.
The Colorado River.
Colorado river
The Colorado River.
Colorado River.
The entirety of the Grand Canyon is in Arizona. The Grand Canyon begins in the northeast at Marble Canyon not far from the Arizona/Utah border. In the west, the Grand Canyon ends just before Lake Mead on the Arizona/Nevada border.
Colorado River.