Easy one.. when the USA PCS spectrum licenses were issued, CDMA was still a mostly theoretical unproven technology, with a few bugs still to be worked out.

GSM at the same time, was already in use in dozens of countries, and seemed (and was) a good choice for digital voice service. As was D-AMPS (without the AMPS portions). Each service area within the USA can have as many as seven digital cellular/PCS carriers (not including the two lower frequency "analogue" bands, on which digital D-AMPS is also now commonplace), each of whom gets to choose the technology they want to use. So it's only natural that several "competing" technologies might be deployed in any given area.

Talk about a nice environment for wireless confusi.. err.. evolution.