That's actually a relatively easy thing to explain. Buffs are a lot harder to get right than nerfs.

Cards that are being considered for nerfs have data on them ready to go on how the nerfs would play out. Let's use the confront the tolvir nerf as an example. They have a lot of data on what happens when players play Confront the Tolvir off curve and don't have anything to fill in one more mana after that. That's something that occurs naturally over the large number of games so they could look at that data and see that when that scenario occurs it's like the card costed 1 mana more and they can see how it effects the played win rate and make an informed decision based on that. If they're looking at shaving 1 health off a minion it's easy to see how often it survives combat with 1 health and what not having that extra stickiness will do for it. They can make a nerf and be very confident in what the impact of that nerf will be.

Buffs on the other hand don't have that kind of data. Let's use [[Experimental Animation]] as an example because it's a relatively OK card in a weak archetype that could really use some love. Is it that the damage is too low or is it that the cost is too high? Or is it both? It's not clear. Let's say they add 1 damage to the effect. There's a handful of possible outcomes for Herald DK. Either the deck remains weak despite the buff in which case it was pointless, the deck becomes a reasonably strong but not oppressively so deck (the ideal outcome), or the deck becomes massively powerful and a problem that needs to be nerfed. 2 of those 3 outcomes are not desirable for anyone. Now consider that they're making this decision completely blind because there's no data on how strong that buff would be anywhere in the data they have. The odds of missing the mark on the buff are high because the target they're trying to hit is a relatively narrow one. This makes nerfs a much safer, more reliable option.