So its obvious that Miku is demed by Sega Japan to be more of an asset to push in the west than Sonic is...
I think they are promoting the characters differently. Miku can really only be promoted by events such as David Letterman. Not only do they not have full control of what can be done with the character she really doesn't make it easy to promote in different ways.
Sega has (what they consider) big plans for Sonic in the upcoming time frame. They have new games releasing, a new series, an upcoming movie. I realize that all of these things are being handled effectively outside of Sega's grasp, but they for some reason have a thought that this is what is best.
Sega has definitely gone more the role of a publisher and IP library than creator in the past years. With the days of mobile gaming about to eclipse retail (if this hasn't already happened in some sectors) I can't really argue they are making bad choices.
All the same it saddens me that Sonic and other Sega characters are being left high and dry. I fear that we may never see the passion and creativity that Sega last displayed in the Dreamcast era.