Bandwidth limitations are one possibility.
Another alternative influence that AT&T may have had is that by offering a $29.99 unlimited data plan with no contract to this device would render pointless many people's need for more expensive voice plans. This might especially be true with users who would couple Google Voice and Skype together, presuming that Skype went ahead and created an app with video conferencing.
Possibly the inclusion of the camera was a negotiable item between Apple and AT&T and allowed AT&T to offer the $29.99 data plan. It's speculative, but perhaps with the inclusion of the camera, which may have also decreased Apple's margins, would have come with a higher cost data plan making the overall package less appealing.
Also a factor in this equation might be the effect that a VoIP enabled video conferencing device on iPhone sales.
And perhaps yet another a factor might be the release of an iPhone with the much desired front facing camera specifically for this type of function.
Finally, it may have just been the case that Apple's stringent product pipeline was unable to include the camera at the desired specifications or that acquiring cameras at the correct cost was not possible for the product launch.
Along with others, I'm keeping open the unreasonable and unfounded hope that the iPad will include one when it launches.