Human dwellings depend largely upon the surroundings and the climate of a places.
When a house is built in the mountains, the roof should be able to bear the weight of the snow and make rain water slide easily. Thus, steep roofs are made of steep slopes, so that after a certain point, the snow slides off to the ground and prevent the accumulation of snow on the house tops.
Below Steep Roofs info from Wikipedia (Read More):-
Some types of roofing, for example thatch, require a steep pitch in order to be waterproof and durable.[1] Other types of roofing, for example pantiles, are unstable on a steeply pitched roof but provide excellent weather protection at a relatively low angle. In regions where there is little rain, an almost flat roof with a slight run-off provides adequate protection against an occasional downpour.
http://en.wikipedia.org/wiki/Roof
