Very little HOTAC at Goose Bay, plenty in Gander but occasionally fills up with various conventions, etc.
St John's has lots of HOTAC and good airfield facilities, but not the best of places from a weather point of view - frequent low cloud and also wind shear warnings on the approach plates if the wind is reasonably strong.
Having operated to the area for many years and without knowing all the details of the weather, I would suggest a diversion to Gander would be very logical and they were just unlucky with the HOTAC (or lack of!).