I can only think more airlines don't fly there becaus it's a very small market. Icelandair are only a success because of their skillfully marketed "Mid-Atlantic Hub" Keflavik is in a perfect position for Europe-USA connections, as you never need backtrack like you would if you flew BOS-JFK-GLA for example.
Many of Icelandair's passengers don't go to Iceland, they simply change planes and fly on to Europe/USA. Other airlines couldn't operate a hub in Iceland so this market has relatively little to interest them. Icelandair have a very modern fleet and good service - add that to almost total market dominance - you can see why airlines would focus on other larger markets that would be easier to enter!