In 1979 Arizona asked AASHTO to extend the US 93 designation to Phoenix; following is the route description from the actual meeting minutes:
At the time, US 93 ended a few miles north of Wickenburg, at its junction with US 89. So if that proposal had been approved, then the road between Wickenburg and Phoenix would have had a triple US route concurrency (US 60-89-93), and possibly that is why the idea was rejected by AASHTO's US Route Numbering Committee.
About a decade later, in 1992, US 89 was decommissioned throughout southern Arizona, so US 93 was extended down former US 89 to its junction with US 60 in Wickenburg. But instead of just terminating US 93 in Wickenburg, that would have been a good opportunity for Arizona to re-submit their proposal to extend the US 93 designation all the way to Phoenix (so the highway's designation would have changed from US 60-89 to US 60-93). But it appears that they never tried again.
At the time of US 93's 1992 extension into Wickenburg, traffic used Tegner Street through town (as US 89 had done). But in about 2010, US 93 was moved to the current bypass that runs just to the east of Tegner. The new junction with US 60 was configured as a roundabout, and it was arranged in such a way that Phoenix-to-Kingman traffic does not even have to enter the roundabout. This reflects the fact that most traffic through the area is traveling between those two destinations, whereas US 60 between Wickenburg and Quartzsite is only a lightly-traveled highway. This makes it even more appropriate that the route between Phoenix and Kingman (and Las Vegas) should be designated with a single number (rather than switching between US 93 and US 60 at Wickenburg).
Typically I am opposed to decommissioning US routes, and I will grant that Arizona DOT has done a good job of signposting US 60's overlaps with I-10 and I-17 through the Phoenix area. However, very few travelers use US 60 to get between Phoenix and Quartzsite, because it follows a 20-mile-longer routing through Wickenburg, and it adds an extra hour of drive time compared to I-10. So given today's highways and traffic patterns, US 60 west of Phoenix is kind of pointless. It would make more sense to:
More about US 93's current and historic endpoints in and around Wickenburg can be found on the main US 93 page.