Why isn't it? Isn't playing a big role on a team - bigger than his rookie counterparts - part of judging the most outstanding rookie?
This Eichel comparison doesn't compute b/c there more teams that rely on their top C (or top player) like Buffalo does with Eichel. While there are a minority of rookies relied upon as a 1st line player like Heiskanen (& Pettersson).
Heiskanen's EV TOI was better for the whole season, not just 1/3. Also more defensive starts over the full season, not just 1/3. On a team that made a night & day turnaround defensively. He was a massive part of that.
Dahlin was a very good top 4 D on the Sabres, and a better offensive contributor. But it wasn't a massive gap. Where-as I think being the #2D at evens on a playoff team is pretty impressive for a first year player.
You can bring up some advanced stats, but that's playing a bit lesser role against a bit lesser competition. I think if Dahlin & Heiskanen played on the same team this season Dahlin would be on the 2nd pair playing behind Heiskanen. Miro has been extremely reliable for Dallas.
FTR its not a massive gap. I'd have Miro #2, Dahlin #3 & Binnington at #4 if I had a vote. Dahlin's first season as such a young player was very impressive.