Well, I don't know the official reason, but I remember back in Office 2000, we were getting feedback that customers, and especially CTO's of large organizations, were complaining that the easter eggs may be causing code bloat or somehow making the application more secure because of the potential of rogue code (in their opinion).  So... that may have contributed to it.

<SelfPromotion>
If you've got Office 2000 and the web components installed, you can find my name in the Spy Hunter-like game that is the easter egg.
http://www.eeggs.com/items/8240.html
</SelfPromotion>