For an in-depth take on this, check out:
In a nutshell, device admin was introduced with Android 2.2 as a means of granting admin permissions to applications. Any number of admins can sit on a device and have excessive, often unnecessary control. It has been widely abused by PHAs (malware, etc) and is very limited in scope of capability, leaving each OEM to build upon it in a fragmented and difficult to support manner.
Android Enterprise takes a more sane approach to device administration by permitting only one owner on a device at a time with scope for task delegation to other apps as defined by the management server, this approach is fundamentally more secure and easier to support as the APIs are universal cross-OEM.
Add in features such as no Google account management, silent app distribution, managed system updates, simple provisioning and streamlined enrolment, and it’s easy to understand why Android Enterprise is basically better in every way.