I don't know anything about WiMax, and weren't the sales of the EVO 3D pretty lackluster?
My understanding based on Evo 4G owners is that there were a number of factors at play, however sales were not as lackluster as people think. They were lower than expected, but that is partly because they were comparing them to Evo 4G sales numbers. The Evo 3d is actually one of the most supported phones you can buy in terms of users and developers, that doesn't happen when only a few phones sell.
The reasons I have heard though, is that the Evo 4g was (and still is) a very nice phone that Sprint sold a TON of them, it was a phone people wanted. A year later, when many would have upgraded, Sprint had changed the upgrade policy, making it so most of the Evo 4G owners would have had to pay for it. Most who owned the 4G felt it was still plenty fast, had a nicer look and format and it was already paid for, so they skipped the upgrade. In fact it's only now with the new EVO LTE and with many now poised for their upgrade that 4g owners are starting to consider giving them up. Even still, some still refuse to (yes, they really are that nice).
As for new customers, it didn't help that the 4G was made in such mass quantities that you could still get them for quite a while and many saw the 3d feature as being the primary selling point and only a gimmick. As the 4g left shelves, Sprint brought in other HTC phones (without 3d), close enough in specs and at a lower price. Then to top it off, the Iphone was brought in. Despite all of that, it still managed to sell quite well, just don't compare the numbers to the Evo 4G or the Iphone.
Customers just couldn't see beyond the 3d feature to see the phone it truly was, which is precisely the reason it was renamed for Virgin. Was it the best phone on Sprint? Probably not, but it crushes everything on Virgin and Boost currently.