Crypto-shredding is the most likely reason the technician is unable to retrieve legible data from the hard drive. Crypto-shredding occurs when the encryption keys used to access data are deleted or destroyed, making the data unreadable. In this case, it is possible that the hard drive was encrypted (either through BitLocker or another encryption method) and the keys were deleted after the laptop was stolen, rendering the data inaccessible and illegible without the correct keys.