The explanation comes from Android engineer Dan Morrill in a comment on a Reddit article, who says that the Galaxy Nexus doesn't support USB mass storage mode for the same reason that the Motorola XOOM didn't with Honeycomb: there's no expandable memory slot. Morrill says that devices with SD card slots will support USB mass storage. However, devices that only have internal storage like the Nexus or the XOOM will not.
As we mentioned though, this is really only an issue for Mac and Linux users because those platforms don't have native support for the standard media transfer protocol (MTP) used by the Galaxy Nexus. Windows users will be fine, but Mac users will need something like the Android File Transfer app in order to transfer files.
source: Dan Morrill via The Verge