Easy. Gilnean Alchemists have noted a splinter group seems to be tinkering with the original serum that worked in the initial Worgen outbreak. Then the PC goes to check them out, and they’re like 'hey, you’re a Worgen yourself/a Worgen friend/play a game that has Worgen as selectable in the character menus, you wanna try something out? 95% safety, mostly-guaranteed!
Do an optional quest at like 15 or so, bam, tail option. So simple.
Then why aren’t Worgen 8 foot tall like your Lore states? Classic Worgen were a product of the times when Blizzard had no intention of it being a player race. It wasn’t till Cata that they were considered and even then Blizzard was so busy working on a full zone for leveling Goblins they ran out of time and produced a shoddy sub par model for Female Worgen when the concept art we got was so much better looking. If your afraid of furries come out and say it, don’t hide behind the false pretense of MUHHHHH LORE. The Lore is Dead, Long Live the Lore but Blizzard killed it long ago with all the retcons and changes.
There were many things that were done incomplete during the cataclysm. I would not be surprised if tails had been removed for lack of time or the means to make a satisfying animation.
Worgen was one of them. And a tail will not grant more DPS advantage or advantage over any race.
Having an upright posture did not give a better performance to orcs.
A tail will only improve the appearance of worgen, which has already been improved (unless you are part of the portion that liked the old model and wanted something more fierce, but never explained what that more fierce would be) and can be more polished with an optional cosmetic, because there is no part of the lore that says that worgen cannot have a tail, especially when Goldrinn does.
A tail will give worgen a more wild and animal look.
As would a lore friendly size of 8 feet tall. The whole idea behind Worgen is they are monsters. They are not a cute race like the Alliance is used to and if the HA quest chain is Lore Worgen can still pass the curse on as Tess Greymane says she will have the champion do. How that a new Worgen would control it without the proper rituals is another unsolved mystery all of its own but we will see should that time come.