This is completely my own guess, here.

Night Elves have a very clear and obvious bond with sabers, from the Wintersaber trainers to the fact that they ride sabers and hunters begin with one. While Lo'gosh possibly taught ferocity, everything about him was to just embrace the ferocious spirit, and he probably wouldn't mind what form the druid took to do this in, as long as they were properly 'rar' about it. The Night Elves found it easier to bond with the cat form, and could still be properly fierce, Lo'gosh wouldn't have a problem with it. There's also the possibility that those original followers of Lo'gosh who eventually created the whole worgen thing were seen as -too- fierce and unbalanced, and the cat wasn't as crazy go nuts thanks to it's discipline needed to use stealth and was more balanced and a better choice to be fierce in without going insane.

Lore states that the Tauren probably learned from Cenarius back in the day, but they forgot somewhere along the line, and it took Arch Druid Hamuul Runetotem to walk up to Malfurion and go 'Teach me, damnit' for them to get it back. Since the current Tauren druids learned from the Night Elves, their form is a cat as well, but looks more similar to the familiar 'mountain lion' types that the Tauren are used to from Mulgore.

I personally like to think that the earlier Tauren druids took on a wolf form, as they'd have had an affinity for them, but when those original druids were gone (maybe a tribe that was wiped out by the centuars before Thrall got there to help?) the remaining tribes lost the ability. 'Runetotem' sounds like a tribal name that was once a family of shamans, who had once worked with the now gone druids, and who were subjected to a profession change by the head of their tribe, Hamuul.

Ingame wise, it's probably a design decision to not confuse people since Shamans have spirit wolf, which lore wise was explained by Orc shamans having been close to wolf spirits and wolf spirits generally representing the 'wilds' in Draenor, and there are no cats in Outland. And I -just now- realized that...the ones in the Netherstorm are arguably put there by the Etherials in their fancy little domes, I dont think the creatures in the eco domes are native to the planet, I think they're just whatever the Etherials collected that looked cool, because of their particular nature. Tauren shaman, if not learning from the Orcs, would just have grown close to the wolf as a powerful nature spirit any rate, and I'm fairly certain that the Trolls learned from the Orcs. Goblins, we know, just took it up because of good old Goldmine understanding the whole idea of 'bargains' with the elemental critters as just another contract. Dwarf shamans make sense having wolves with how there are an awful lot of wolves in the areas where the Wildhammer lived, too.

tldr; Night Elves learned being 'fierce' from Lo'gosh but didn't really know wolves thanks to where they lived so translated being fierce into being a cat (or the wolf was too unbalanced and prone to rabid while the cat and it's ability to stealth was more balanced and focused), and current Tauren druids first learned druiding again from Night Elves so picked up the cat form too.