I think it's not about a comparison to other cards on the restricted list. They clearly aren't of comparable strength to each other in the first place, and there are unrestricted cards a good deal more powerful than several on the list. The decision to restrict a card has to consider the full context.
The first question would have to be if Chaos with an unrestricted Sorcerer is leagues ahead of the other factions. If it is, a restriction might lead to balance. If it's not, if it's on equal footing with the others while it has the Sorcerer, why make it fall behind by taking away one of its strongest cards?
Assuming Chaos has an advantage over all other factions with the Sorcerer, a restriction might be in order. But a restriction would mean Chaos would have to choose between it and Warpstone Excavation. In the case of almost all cards on the restricted list, Excavation wins these contests. It would require testing, but I can imagine Chaos loosing too much if it goes with the Sorcerer over Excavation. Which means you would have effectively banned the Sorcerer, because Chaos has to choose Excavation. And with Excavation but without the Sorcerer, it might not lose as much, but it would still become less powerful, maybe to too high a degree. So instead of the restriction leading to a balanced field, it would only lead to a new form of inbalance, one where Chaos is behind instead of ahead.
It's not as simple as saying this card is very strong and shows up in almost every deck (of its faction). If that were enough, a lot of cards would be restricted, only for new best cards to emerge that take their place.