The appliance is Cisco-branded but it is built on a general purpose commercial server platform (an IBM server in this case).
General purpose servers have commodity hardware designed to meet a wide range of implementations and use cases. For that reason, they include things like a second NIC - it may be useful for some uses and not used at all in others.
Cisco has built Prime Infrastructure on the Application Development Environment Operating System (ADE-OS) shell over top of Red Hat Enterprise Linux (RHEL). One could just as easily ask why you can't use iptables as a host-based firewall or run an ftp server on the Prime Infrastructure server since RHEL is running on it.
The answer is that Cisco pre-packages the OS build and decides exactly what hardware (and software) options to support in order to reduce the number of variables in the installation and make the product more supportable and servicable. There's always a trade-off in features when doing that. They make a business decision where to strike that balance at the time they develop the product. It isn't always the best decision for every customer's needs but that's how it is. If they hear back from enough customers that a certain non-included feature is really important, it may get rolled into a future release.
