These gun skins actually take quite a long time to get made, and the team was making weapon design changes internally all the way up until the release of our Friends and Family test build shortly before Closed Beta. At the time we made the call to make a lot of these skins, the Phantom was definitely more of a second fiddle niche weapon defined by its silencer and more controllable spray, with the Vandal being the default premiere rifle. The Phantom's since been buffed up to be similar in power levels to the Vandal, but the skin representation calls trail behind that due to development time.
It's definitely not some big brain marketing scheme like the posts below imply... the game's just constantly evolving, and it takes much longer to concept out and then develop an entire skin line than it does to tweak weapon numbers in development. We're not intentionally withholding all the dope Phantom skins we've got locked away, these things just take awhile to make!