The main reason is that the house collection has been much more systematic, and the exhibits, activities and guided tours even more so.
There is also the permanent historical exhibit "Slowly, the land became our own" in the indoor section, which takes a view of Norway emerging from the ice age, through early settlement, nation building and colonialism to liberation 100 years ago and into modernity. Very interesting - and gives you a skeleton background on Norway.
There are many fun things and unique things at Norsk Folkemuseum, too, especially in their city department, but if you will have to choose one, go to Lillehammer.

