Perhaps number of speakers? Icelandic only has 320,000 speakers. I suspect it would be easier to control the direction of a language with that many speakers compared to French's 200,000,000.
I would guess the relative isolation of Iceland for a very long time is also a factor. While the French constantly mixed with pretty much all European peoples during medieval times, receiving an influx of things and concepts in need for a word, the number of people crossing the borders of Iceland was very limited, for practical reasons.