The main reason that I can think of for why most companies do not use disk encryption is that many people forget their passwords. When that happens, they are completely locked out of their laptop. Ask any company help desk what its most common problem is, and they will tell you it is password resets. Now, many good encryption programs for laptops provide what is known as escrowed keys, so that a help desk can get a user that forgot their encryption key going again. The problem is that the process of recovery takes longer and adds even more work to usually overworked help desk staffs. So rather than add to the cost of user support, the companies simply expose all their data. Seems like a foolish risk trade off to me, but you asked why they did it, not whether I thought it was a good idea not to use encryption.
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