What feature of a server makes it a potential and likely target of hacker attacks?

The feature of a server that makes it a potential and likely target of hacker attacks is it network capability.

Hackers are interested in attacking as many computers as possible.  Therefore, their best target is a server that serves the networked resources, computers, and systems.  From that point, their malwares can be distributed easily and smoothly to unsuspecting computer stations so that they in turn start replicating the malware and also malfunction.

Explanation:

Bear in mind that a server manages all network resources.  The storage capability resides in a server.  The security settings are usually installed in the server to serve all networked devices and systems.  It is from the server that all systems are administered.  Servers are usually dedicated to their server roles and nothing else.