<p>We are currently having two UI design for internet and intranet the underlying reason being is that they both serve different purpose so they have different UI design.</p>

<p>But however users want to have a more synchronized look and feel between internet and intranet.</p>

<p>What are the reasons you all can think of in have different user interface/outlook for internet and intranet?</p>

There may be
more information
available on intranet.  Also the tone of the message might be targeted differently but you'd want to ask marketer about that. There might be less of a "sell" to the intranet.
Often
authentication
would be different and even integrated with domain.
Big difference for intranet is that often you don't need to design to the
lowest common denomitor
, i.e. what works across many browsers.  You might even be able to presume other things about the user's environment you would never be able to on the internet, therefore being able to develop against a
known target
.  Example, you might be able to assume every user runs linux, FF 3 and has flash or alternatively windows vista, IE8 and silverlight.