Good point, but Wii had a bit more support back then, now it's a desert.
The Wii was never that strong in Japan. The Wii was falling every year, from 3.68 in 07, to 3.02 in 08 (despite sales increased in Others and the US that year) and 2.02 in 09. The Wii was never doing that strong in Japan despite the DS did much better (7.21, 4.26 and 4 in their respective years). Both the DS and the PSP are are 6 years old, so they are going to fall in sales, and there are fewer big titles. Iwata himself said Nintendo needs to release more social phenomenon.
The PS3 did better in 2009. That's all.
Nintendo can only do so much on their own. Without the help of third parties that will be impossible, because they will have to go through long droughts and momentum is then lost and people just moves on to the consoles where the games are, like it happened after 2008.
Again, third party for the Wii has always been bad. The only titles that made a strong effect were the Mario and Sonic ones and Just Dance. Third party did not prevent Nintendo from selling insane number in 08 and they probably are not the reason for slow sales now.