There are a few reasons why CDMA is becoming more prevalent in Japan and the US, and some other countries (US mainly though)
I will go over the Pro's and Cons associated with each, and then I will shut up.. I promise.
CDMA as it is used now (CDMA2000 and CDMA EvDO [ Evolution Data Optimized]) is a great way to "unclutter" the RF space (Radio Frequency space) in communications. here is why
GSM: uses multiple frequencies in a TDMA (Time division multple access) scheme for the uplink and downlink of voice and data.
Example: you have a cell tower, and it serves an area. There are 6 people utilizing that cellular tower for communications. They are all using GSM850 (GSM850 and GSM1900 are what is used in america for TMOBILE and ATT for Voice and SMS Bands)
Because GSM VOICE uses TDMA for the technology base, when person 1 calls person 2, they are using 4 different frequencies on that tower. IE person 1 is using 855.2MHZ for his Uplink and 885.2MHZ for his downlink (note this is just theoretical, and there are multiple frequencies allocated to providers, etc.) and person 2 is using 875.5 uplink and 865.5 for his downlink.
Now throw in guardbands so you dont get frequency bleed-over, etc, and you are using a great deal of RF space to communicate.
NOW for CDMA
say you have a CDMA tower, a common frequency range is 450mhz for CMDA.
Because CDMA phones use Code division multiple access, you and another caller, or another person on the same "cell tower" can be on the same frequency, because a "code" is applied to your call so it can be differentiated when transmitted and received.
so caller 1 and caller 2 can be on the same 450mhz frequency channel, and because their transmissions are coded (RX/TX) they can be differentiated even though they are on the same frequency. This is the beauty of a digital signal such as CDMA2000 and CDMA 1xRTT EvDO
Advantages of CDMA
Less frequency space use, therefor cheaper for the provider to lease and use and deploy
Faster data (on paper, but the USA sorta screws it up)
Disadvantages of CDMA
No Data/Voice at the same time (even on some 4G phones, with WiMax/LTE switch on, you still cannot browse and talk at the same time.
No actual encryption (because CDMA uses a digital signal with a sort of "frequency hopping" scheme, they say its "built in security". Its not. :-\\
GSM advantages
WorldWide use
Simultaneous Voice and Data (on most networks)
Faster data throughput
Good encryption (current standard in america is A5/1 and A5/3)
I hope this helps, or maybe I am just too talkative.
Anyone correct me if I am wrong/missed anything.