It's because their usual tactics may not be as effective
I think Triskelion is a better card with your Commander than Walking Ballista, but both can go infinite with Mikaeus iirc, but you'll need an extra piece with Walking Ballista because Mikaeus makes it a 1/1 base creature, so it won't die because it has no counters, Triskelion can ping itself and still have a ping for something else, meaning you can combo off easier with it, so I'd go with that
Eldrazi Confluence Its hard to argue with how flexible and powerful Eldrazi Confluence clearly is
He's a top commander for those still learning how to play Magic The Gathering's most popular format, since you can experiment to your heart's content, and it's hard to go too wrong
The biggest boon that this starter deck carries is that it comes with two copies of Dracule Mihawk, a generic card that can be splashed onto just about any green deck that plays a handful of Slash attribute cards