To make a long story short, it was too large a space for any of the known type of domes be able to cover the space. For over a hundred years, the cathedral of Florence, Italy, had been sitting essentially without a roof. The building of the church had begun in 1296, but when it came time to build the envisioned dome over the area, no one could figure out how to do it. The area to be covered was 150 feet across, and the dome itself would start at the top of the 180 foot high church walls. The ancient Romans had been able build domes of this size, but the knowledge of how to do so had been lost. After 100 years of rain damage to the interior, the town officials of Florence decided to have a contest to design a dome that would work. A goldsmith named Filippo Brunelleschi proposed a double-layered, dome-within-a-dome. He'd spent some time studying the architecture of Rome, and eventually the dome was constructed. This being medieval Florence, there was a lot of behind the scenes drama that went along with the planning and building. The link below tells about this in great detail. I have been to the Duomo, and it is indeed extremely impressive.