Cost and convience. If I were to build a round house structurally sound enough to have a 30 ft. 4 or 6 bladed Holland type mill, I'd need to constantly be adjusting the vanes to keep the blades spinning at a fairly steady speed. Also I'd need to manually move the blades to aim them into the wind. If I were to get 20 Kw average output, at 10 cents per Kwh, my time would be worth 2 bucks per hour. If a strong Gust came along, and no furling happened, the house could collapse. Oversimplified obviously! Gary D.