It may be something to do with SIM usage. Possibly the CDMA carriers issue SIMs that doesn't do CDMA authentication (that bit being done by another set of IDs which are not stored in the SIM) - the SIMs they issue only being for GSM roaming.

It'd make it harder for a general phone to roam onto CDMA if there are no suitable credentials in the SIM.

However, I managed to avoid CDMA when I was at apple and so don't know the technicalities of this. I've not seen any world-phone providing GSM carriers talking about their phones roaming onto CDMA either though.