Can anyone explain why they feel the need to nerf popular talents rather than just buff the unpopular ones?

In this specific case, I really think it's what they said in the patch notes: he's an auto-attack hero who had more burst than they wanted. Moreover, I think he never landed where they wanted him to be when they designed him. It seems like their image of what Greymane should be is not a diving melee assassin (Illidan, The Butcher) but a ranged assassin who can dive in and finish off weakened foes. He's "supposed" to use his cocktail to soften the back line and then switch to Worgen form when appropriate. They want the cocktail to be a little hard to use but have a big impact (so to speak) when a skilled player lands it. This is why they changed Incendiary Elixir so that it doesn't explode automatically.

The other side of that is the fact that in worgen form he has no defenses (well, optionally, two block stacks). In general if a hero has an "all-in" engage mode, his damage will be very high to make the reward worth the risk. The Butcher has no escapes but can at least heal himself. Illidan heals when he attacks, evades auto attacks, and has Q and W to escape. Pre-nerf Greymane was exposed but did major damage. Often teams would take double supports to compensate for this -- Rehgar + Tassadar being common. Is his melee damage enough to warrant an investment by the team to keep him alive now?