Simple answer? Escalation. Suppose the dev team never nerfed any of the commanders, even when they release one that’s more powerful than they had planned, and instead continuously buffed the other commanders until they were on the level of whichever one was strongest. What happens then? The difficulty of the missions drops down to the point where people get bored because they’re so easy. So, naturally, they have to buff the AI units as a result. This cycle repeats every time a new commander is released, or even when one if buffed and they overdo it, until every commander is spitting out full economies and free supply right from the start of a mission, the first attack wave arrives at minute 1 with Hybrids and basic Marines and Zealots have triple-digit health and damage. At some point, the dev team has to draw a line and says “this is the limit of how strong players should be, this is the limit of how strong the AI can be” and get the strength of both as close to those lines as possible through both buffs and nerfs, to ensure that every co-op match offers the opportunity to both make the players feel powerful and offer them a reasonable challenge. Zeratul, Tychus and Mengsk were over that line, so the devs reeled them back a bit to try and bring them more on par with the other commanders.
And that’s certainly a noble sentiment, but this is like getting kids to eat their vegetables: they don’t understand that it’s good for them in the long run, all they care about is that it feels bad for the short term, so they are going to complain until their faces turn blue. Sure, the devs could compensate players every time they nerf something, but that’s like bribing those kids with candy so they’ll eat their greens, it makes them happy for the moment but it sets a bad precedent and they don’t learn what they needed to understand in the first place. Better to just give it to them straight and talk them down as best you can, at least then it has a chance to sink in. Eventually.