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There are a number of reasons you could name.
Historically, you could point to the experience of the Latter-day Saints in the 1800s which was one mass exodus after another. Emergency prepardness was life or death.
Culturally, there is a strong emphasis on hard work and temporal self-reliance (probably carried over from those early pioneer days).
Pragmatically, it makes good sense.
Doctrinally, they believe they live in the last times (latter days), which are promised to be so tough that even the righteous barely survive.
And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
Matthew 24:22
Many times, "emergency preparedness" is interpreted as "prepare for the nuclear/end-of-days/zombie apocalypse" by stockpiling food and weapons in bomb shelters.
In reality, emergency preparedness is much less dramatic. For all the decades Mormons have stressed preparedness, there's still no apocalypse.
But loss of employment, local natural disasters, illness, death of a family member, etc. are common. Mormons prepare for the proverbial rainy day.
Last month, the president of the Church said (and similar quotes can be found many places):
We live in turbulent times. Often the future is unknown; therefore, it behooves us to prepare for uncertainties. When the time for decision arrives, the time for preparation is past.
In addition to individual preparation, the LDS Church itself has orchards, farms, canneries, storehouses, and the largest cow-calf ranch in the US. It puts these to use in its welfare program, charities, and humanitarian aid services.
For individuals, the Church emphasizes:
Employment The LDS church runs employments centers, open to members and the public to help people get job skills and find employment. Members are taught to get as much education as possible, and the Church has the Perpetual Education Fund.
Avoid debt Education and a house have been listed as reasonable exceptions, but other debt, particularly consumer debt, is strongly discouraged.
Food storage The LDS Church used to recommend two years. Now, they recommend three months plus "extended" food storage, when feasible.
Health Follow the Word of Wisdom (LDS health code: grains, fruits, no alcohol, no tobacco).
The Provident Living website is a good official LDS source.
Of course, there's always that guy with his bomb shelter, 200 gallons of canned food, and private armory, but the LDS Church isn't suggesting that.
Being from Florida, I've been through a number of hurricanes, and it continues to amaze how many people will wait until the last possible second to prepare. Katrina was probably the most extreme recent example of this. Preparing to take care of your needs, rather than waiting for FEMA to do it for you, is the right approach.