Again, it's more of what Sony and MS have currently done right than what Nintendo has done wrong. Wii is simply following the natural console life cycle, while the HD consoles have had their lives extended with console redesigns, multiple price drop (especially PS3), and their own motion controllers. If they had not had these features, they would be where the Wii is right now or worse.
You're correct about the other reasons. Third parties have frozen the Wii out, essentially cutting its life short by not filling in the gaps in the software library. Nintendo can't pull the weight 100% of the time, and now that their focus has shifted to 3DS, there is no one left to make Wii games. I'm sure Nintendo will soon shift back to Wii support soon though, once 3DS is released.
As I've said, Nintendo hurt themselves when they decided to make their biggest games of 2010 Super Mario Galaxy 2, Metroid Other M, Goldeneye, and DKC. Only DKC was really a potential system mover. They had NO expanded audience games, no new ideas to draw attention, and only one bridge game in DKC. The rest were purely hardcore games for a niche audience. That is not going to cut it. So the bad 2010 (speaking in sales potential here) hurt the console and carried over to this year. To my knowledge, they had no games that sold 10 million copies during all of 2010, besides what was already released. 2010 for Wii may have been a strong lineup to us gaming message board dwellers, but remember that we only represent a very small portion of the gaming audience.
Anyway, people should stop making a big deal about Wii being in last. It is not surprising when you have no third party support, and your first party support has shifted towards a new console. People seem to forget that PS3 and 360 are not invinsible either, and will soon follow the Wii's downward slide.