They don't have an offering on Verizon and some others because they don't have a CDMA version of the phone. With the networks soon moving to whatever 4G is, there's not much point in Apple making a CDMA phone now.
4G is not really a replacement for CDMA. You don't need a 10/1Mbps connection to send highly-compressed voice, and even as 4G is rolled out, the networks are not going to decommission their 3G equipment. (Consider GSM, where you have HSDPA/UMTS, EDGE, and GSM all as the "same network". WiMax will supplement CDMA, not replace it. And I assume the idea is to move bulk bandwidth from CDMA to WiMax so that the CDMA network has more call capacity.)
Since Verizon is doing an LTE overlay on top of their CDMA network it seems likely CDMA will be around for another 5-10 years. At some point it will exist simply for redundancy in the same way GSM carriers simulcast 2G and 3G today. I don't think we'll see any pure LTE-only phones for a few years. They'll all be LTE/CDMA, LTE/GSM, or LTE/GSM-CDMA capable. So it probably isn't a total waste for Apple to devote the resources to CDMA now with plans of moving to LTE/CDMA or LTE/GSM-CDMA in the next couple of years. I think either way Apple will have to support CDMA in some form if not to reach parity with other LTE/CDMA handsets that will be more reliable and may have better coverage in rural areas (ie, some coverage versus none)
