In a message to backers of the Robin, Nextbit CEO Tom Moss explained why the company made this decision. Originally, Nextbit was not going to even release a U.S. CDMA variant during the Kickstarter campaign to help fund the Robin, but the company got a lot of requests to add that version in with the already planned GSM phone:

As you can imagine, we were in a rush. The Kickstarter campaign is only 30 days long, and it wasn't until the second week of the campaign that we realized the demand and started the investigation. Because of this, we had to go with the best information we could get before the campaign was over, and over time it turned out that this information was not accurate. What people at the carriers, in good faith given our need for quick answers, thought would take "weeks" has turned into "months". What they thought would cost "hundreds of thousands of dollars" has turned into "millions". And we're still not there. The goal posts are still being moved, and at this point, we think it is better to cancel this version rather than continue to try and make progress with no clear answers to offer regarding when we would actually be able to ship.

We first realized there was a big delta between what we originally thought and reality in January. It should have been sooner but to be honest a big part of the problem is how long it takes just to get the right information (and in some cases, what we were doing was a "one-off" for the carriers and they didn't even decide themselves yet how it would be treated until as late as a week ago). Even then, we believed we could recover and launch in March. Then new information came up and it became April. We really thought that would be it, but yet again, we were wrong. We should have known better. We were not sufficiently doubtful of what we were told given everything we already knew from our experience at previous companies. We were too optimistic, too bullish, and as a result we have to deal with our biggest fear, disappointing you, our supporters. This is bad for you, and this is bad for us. The best we can do is send you this explanation with our sincerest apologies, and try to make sure you don't suffer any financial loss in addition to the disappointment of not receiving a CDMA Robin.