Perhaps Three thought it was a good chance of an exclusive, given Vodafone and O2 couldn't take it - and EE may well have said no given it has 800 launching soon too. How many exclusives has Three secured over the years (bar the odd limited edition colour)?
Let's be fair, the phone still works well. It holds a signal well, has excellent battery life, a VERY good camera, a lovely screen, and is dual-SIM.. plus it has 32GB of internal storage (and if you don't want to put a second SIM in, then you can chuck in a 128 or 200GB memory card).
So, for the money and IF you're happy with the current 3G and 4G offering, and knowing that 4G at 1800 will still continue to roll out and offer higher speeds (most of the time) then it isn't the end of the world. Perhaps.
But, Three should have been more open and should be making it clear right now which phones are 'VoLTE ready', with some sort of explanation of what it is (as all networks will have to do eventually when launching). For one thing, saying it's coming may well get a lot of people to sign up to Three, knowing that in a few months time the service could be drastically improving.
Then, you could make an informed decision.
However, imagine if Three had insisted on Huawei giving them a version with 800MHz support. It would have been an awesome offering, in many ways up there with the S6 and G4. Okay, no QHD screen (but the screen is still excellent) but with a camera that for most uses is still excellent - and arguably gets better results in mixed conditions than my Xperia Z3!