The main possible reason is because most common Linux distribution's IDENT or "Identity" servers can't deal with IP Masqueraded links. No worries though, there are IDENTs out there that will work.
Installing this software is beyond the scope of this HOWTO but each tool has its own documentation. Here are some of the URLs:
Oident is a favorite IDENT server for MASQ users.
Mident is another popular IDENT server.
Please note that some Internet IRC servers still won't allow multiple connections from the same host even if they get Ident info and the users are different though. Complain to the remote sys admin. :-)
|( SAMBA ) - Microsoft File and Print Sharing and Microsoft Domain clients don't work through IP Masq! To properly support Microsoft's SMB protocol, an IP Masq module would need to be written but there are three viable work-arounds. For more details, please see this Microsoft KnowledgeBase article.||Up||( DCC ) - mIRC doesn't work with DCC Sends|