"We had to make some trade-offs," senior product manager Greg Sullivan told CNET in the US. "Even Microsoft doesn't have unlimited resources. We had to prioritize doing fewer things, really, really well."
Microsoft, as Pocket-lint exclusively broke is expected to announce the official launch of Windows Phone 7 on 11 October.
Although the news won't affect UK and European customers looking to sign up to the new operating system when the phones start launching later this year, it means two out of the four mobile operators in the US won't be able to offer the handsets.
"For the worldwide market, the vast majority of phones are GSM phones, so we focused on GSM first and then plan to deliver an update that will have great CDMA support in the first half of 2011," Sullivan said. "That's device availability in the first half and we're very confident of that. That's probably a conservative estimate."
AT&T in the USA is expected to be the major partner with T-Mobile offering some support.
It means that Verizon has missed out once again on bagging the latest smartphones, with the US carrier still to stock the iPhone.