Information
Mobile devices not only contain information, they also contain passwords and other credentials that can enable an attacker to retrieve confidential data from other sources the device may interact with. Note that the encryption process requires an hour or more, a fully-charged battery, and that the phone remains plugged in until the process is complete. If the encryption process is interrupted, some or all data may be lost.
Solution
From the MobileIron console, open the Policies and Configs -> Policies view. Under the Security Policy verify that Device Encryption is set to on.