Customizable by WHOM?
If u refer to carrier, CDMA phones are "costomized" to the particular carrier too.
If u refer to end-users, and the availability of GSM flex, monterpacks etc, is simply that worldwide there are more GSM phones and more ppl tinker with it.
As Fdjordan says, u can customize CDMA, it's just not as popular but u can find moRAB.