Because the KEF routing is non ETOPS
AFAIK. There's the Faroes, KEF, SFJ and a couple of others in Greenland all close enought that non ETOPS aircraft can use it.
Etops costs money. Must carry more fuel, same engineer can't check/work on the same system on both engines, I think the pilots might need an extra legal endorsement too?
The rule for non ETOPS is you cannot be more than 60 mins (roughly 400nm for most jets) from an airfield, so it can be done non ETOPS, it just makes it longer than say NCL-YQX-EWR.