They're supporting the 3DS because they think they can make cool games which suit their vision and they can take advantage of a new paradigm, 3D whilst making good money at the same time with a high average sale price on the cartridges.
If it was all about the money they probably wouldn't be in the industry, most of the great developers could have likely made far more money in other fields. If you make an unloved platform (Wii) then its quite likely that its not going to get a lot of grass-roots support in spite of the potential money to be made.
The support for the 3DS which has been pre-ordained is indicative that Nintendo has been able to win over not only the end users but the developers and publishers to a greater degree than the DS and Wii.
Tease.
