根据以下内容回答题：Last December"s earthquakes in the Iranian city of Bam took a huge death toll—roughly 40，000 people——largely because of the collapse of thousands of mud—brick buildings．If a group of researchers in India are successful，the next earthquake might not be as destructive．British and Indian engineers are developing earthquake—proof housing using a cheap，universal material：bamboo． Thev designed a model house built around waterproof bamboo-sheet roofing and bamboo-reinforced concrete walls．To test the structure，the engineers，sponsored by the U．K．Depart-ment of qnternational Development，took it to the Earthquake Engineering and Vibration Re-search Centre in Bangalore，which has a state．of-the-art earthquake simulator（模拟装置）．The researchers shook the house with five successive 30-second pulses，being equal to 7．8 on the Richter（里氏）scale．The simulation was more than 10 times as violent as the Bam earth-quake．yet the house emerged undamaged．“We didn’t even crack the paint，”says engineer Paul Follett．of Britain’s Timber Research and Development Association． By some estimates，more than a binion people already live in bamboo structures．The innovation lies in developing ways to exploit bamboo’S spring．Easily pre-built，fire resistant，and far lighter than steel，bamboo．based structures could be assembled in three weeks and last 50 vears．At five dollars a square foot，they would last roughly half as much as brick-and-block constructions．Follett says the project will follow an“open source”model：“Whatever is developed is freely available for the common good．”

Thousands of people died in the Bam earthquake mainly because__________ ．