Apple Xserve was introduced back in year 2002, after struggling for 8 years time, Apple decided to stop further development on the Xserve. Xserve is a rack size server, which will fit into the standard server rack that normall put inside the server room.
Apple machine was never goes into enterprise data center or server room, at most it sits on top of the desktop of some graphic design companies.
Lacking of software solution is one of the major problem with enterprise server market. Hardware specification, price and performance is a consideration, without enterprise software solution, the hardware machine doesn't works at all, no matter how good the hardware specification.
Apple already have the server operating system which is the unix-base Mac OS X server. One of the enterprise server application that Apple (and other server vendors eg. Sun Microsystem) should looks into is enterprise email server solution.
Enterprise server hardware vendors should work close with software vendors, for example Zimbra. Zimbra is an open source email server, it was acquired by Yahoo and now with VMware. Zimbra works on Mac OS X, but is not a common solution.
Recently I just talks to few IT managers that I meet in the seminar, they use Microsoft Outlook Server as their enterprise email solution. I believe the enterprise email solution market is big enough to fit in 1 or 2 more vendors.
May be Steve Jobs not interested to go into enterprise market.