Why would a FHA loan be required to have PMI insurance?

I am borrowing well less than half the appraised value of the home. I usually only hear of PMI being required if someone is borrowing over 80% of the homes value and then it drops off at a certain point. Why would my loan even have to have PMI and why for the entire life of the loan?
Update: I am going from an 8.2 rate to a 6% but I am self employed and honestly don't make a tremendous amount of money. Could that be it?
Update 2: I am doing a 30 year fixed but my loan amount is less than 28,000.oo.
Update 3: The house appraised out at just under 57,000.oo.
Quicken was the only company that would lend me less than 50,000.00.
Update 4: I don't really have 50% to put down per say. I just am only borrowing less than half the appraised value.
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