There is a long list of reasons.
Networking- Currently their network is based on Novell Netware. Novell do not support their client software on Windows XP Home, based on Microsoft's position - it is not designed for use in corporate or enterprise networks and so therefore they are not doing any work on their client on XP Home.
In the future XP Pro users can move to Microsoft domain/Active Directory based networking, and as mentioned earlier Windows XP Home cannot be used in that environment either.
Windows XP Home cannot be supported in the current networking environment, and cannot be used in possible future environments.
Security- Windows XP Home and Windows XP Professional are both relatively secure operating systems. However, XP Professional has some features that Windows XP Home does not have. First, Windows XP Home does not have file access control, so all users of the machine have access to all files in the same way as on Windows 98. Under Windows XP Professional access can be restricted so that users can only see their own files. Second, Windows XP Professional can be configured to have encrypted file systems that are pretty secure.
Offline features - Windows XP Professional has a development of the "Briefcase" and "Offline Folders" technology seen in earlier versions of Windows which allows people using laptops to automatically "download" their files from a server, work on them while disconnected from the network, and then have them automatically upload when they next connect so that their work is kept sychronised and available both on- and offline automatically. Windows XP Home does not have this. Windows XP Professional also has the Remote Desktop facility which allows you to access your desktop from other machines
System Features- Windows XP Professional has support for multi-processor PCs. It also has additional features for backup and restoration of crucial system files and states so that it should be easier to recover a Windows XP Professional system.
If you would like more reasons, please visit this site:http://www.winsupersite.com/showcase/windowsxp_home_pro.asp
Edited by gavinseabrook, 18 May 2007 - 06:57 PM.