Well, it's true that Björk's fame level exceeds Stefán's (and any other Icelander's), but (1) thanks god, she is unlikely to die in the near future (age 51, no terminal illness); (2) she was born in the capital, Reykjavík, where more than a third of the population lives.
Stefán, on the other hand, is expected to expire pretty soon (this year), and was born in a smaller town called Hafnarfjörður, where he could be one of the most famous former residents (Gylfi Sigurðsson of Swansea might be the other one). And they would like to honour Stefán Karl, the actor; not Robbie Rotten, the cartoon figure.
But since this petition was started by a guy from the UK, I'm not sure how much support they have in Iceland...