Trine is not a big seller. That's all.
It's hardly worth it to make the DLC available for PSN and XBL and they see potential sales in the WiiU version as there isn't much competition on the platform.

http://www.gamesetwatch.com/2012/07/16/xblaregjune2012.png
~50k sales after 7 months on the platform.
PSN might be a little higher since the first one was PSN/PC exclusive, but not by that much.

They should skip consoles all together and make the series PC exclusive.

So many idiots roaming VGChartz nowadays....