I think its just, as others have said, something really poorly handled from the publicity side. People have been requesting Viera for....since ARR? And as early as HW I think is when we saw the concept art of the male Viera. Then more recently you have big chunks of lore about the male Viera that all sounded very much like it was being set up as a way of writing them into the world even if they didn't live with the female Viera. I honestly thought, after reading the lore, that they'd make Viera's two factions male and female and split it that way. They did this odd dance of letting people assume based on their tidbits and refusal to say one way or another, only to then pull a switch a roo with a race they'd never even really hinted at. Another genderlocked one at that, though who knows what excuse we'll have for why they have no females around yet don't go extinct.

Pair that with some general disappointment about the lack of new healers and previous complaints like Eureka and BLU, things do start to compile into irritation that is hard to shake. Having a new healer might have blunted the sting of gender locking or vice versa but they did both and delivered the news at the same time with little to break their own fall. Besides gender locking is just so...regressive and tone deaf. In our game men shall be big strong men and women shall be gorgeous, high heel wearing sex bombs, end of story. None of that is to say they dont work hard...though I will say its got to be way more work to design two separate races with two factions each, and their own lore rather than, ya know, using the lore you have, and making Viera factions the split between male and female. That seems like a much easier task that would have satisfied the player base. I can't honestly think of a good reason why they went the way they did knowing the anger it would cause.