Phoenix has been one of the fastest-growing cities in the nation over the last fifty years, and they responded by building more and more freeways to the point the urban sprawl resembles mini-LA. They spent the entire 80s, 90s, and early 2000s building more freeways in a desperate scramble to handle the growth. Now the lack of urban planning is biting them in the ass hard.
Moreover Arizona, being the Republican state that it is, would not adapt California-style pollution regulations in a million years. Their voters balk at paying taxes so funding any mass transit other than a decrepit bus system for teh poors was a pipe dream for a long time (this is beginning to change now, but they are only now over the last decade starting to build the bare bones of mass transit from nothing, they have nothing like the well-developed subways of the East Coast or Chicago). Worse, a large subset of Maricopa county voters are the type that would deliberately buy gas hogs just to spite "teh libruls" (keep in mind this is the same jurisdiction that keeps re-electing Joe Arpaio even though said sheriff costs the county taxpayers TENS OF MILLIONS in paying out civil rights violation suits).
Oh, and mountains ring the city. They literally form a semi-circle around about the east half of the metropolitan area, there's a small mountain range on the south of Phoenix proper, and I believe there's another mountain range off further to the west. Phoenix isn't called "Valley of the Sun" for nothing. All that means is that pollution gets trapped in there, even worse than LA. At least LA has an ocean on one side.
(Edit: The poster above explained the temperature inversions far better than I can, but the geography described above makes the inversions worse than LA when it comes to trapping pollution near the ground.)
Ironically Arizona used to be a place that doctors recommended for people with respiratory issues, because other than dust there was not much in the way of respiratory irritants. Not anymore!
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