Nintendo has always sucked at marketing their games, and unlike Sony and Microsoft, they almost never offer support for third party games.  Both Microsoft and Sony have included third party games in their advertisements over the years.  Now, maybe the 3rd party companies requested this, and maybe Sony offered.  Whatever the case is, third party games got help from the console makers from Sony and Microsoft, whereas they didn't from Nintendo.

I really don't think that is the case this gen.  This gen, Nintendo has done loads for 3rd parties.  For example, they have localised and published the likes of Dragon Quest IX, trauma centre, samurai warriors, tenchu and probably some more I can't remember.  Nintendo also did the massive ad campaign here in the U.K for Monster Hunter, which is possibly the biggest gaming ad campaign this year after DQ IX, Galaxy 2, Mafia 2, Kane & Lynch and FF XIII.  

Nintendo even let their 1st and 2nd parties do work on 3rd party titles like Genius Sonority on DQ Swords.  I also remember hearing that Retro helped with the controls on Red Steel 2. Wii Motion Plus even lauched with 2 3rd party titles (TW and GST if I remember correctly) instead of a 1st party title.

I wouldn't say that either Sony or Microsoft this gen can rival that.

It might be different in the UK, but in the States, Tenchu wasn't published by Nintendo.  It was either Sierra or UbiSoft (I don't remember), and in the States, Nintendo does next to nothing to help 3rd party companies.  When the PS3 launched, Sony aired several commercials showcasing an amazing number of games, both first and third party.  The Xbox360 launched with a Final Fantasy special edition.  The PSP has a couple packages with non-1st-party titles packed in.  My PSP is a Metal Gear Solid: Peacewalker edition. 

Here, we have some developers who truly believed in the Wii and went out of their way to craft unique titles for the thing.  Regardless of how they may have performed, they were none-the-less unique, of generally decent quality, and again--they believed in the Wii.  Titles like MadWorld, Deadly Creatures and Tenchu--and they got the cold shoulder from Nintendo.  You'd think the company would be interested in supporting 3rd party developers that clearly believe in what they're doing.  Again, Sony and Microsoft dumped money into promoting third party games, but Nintendo didn't.  Sony's early PS3 ads would typically feature something like 4 games shown off in one commercial. 

At least here in the States, Nintendo has always seemed pretty crappy at advertising--especially on television--their games, and they never help third party companies get the word out.  They barely promoted Eternal Darkness at all aside from announcing a short film competition, but without television ads and more promotion behind it, the game fizzled in the sales department.  I think I saw the game advertised on TV a total of one time--and I hate to admit it, but I watch a lot of TV.

Microsoft, on the other hand, dumped quite literally, half a billion dollars into just promoting the original Xbox, and despite launching with an arguably very unimpressive library, the Xbox outsold the GameCube.  Over here, DQIX and Mario Galaxy 2 aren't advertised anywhere near as much as Kane & Lynch 2 or Mafia II.  I saw two ads for DQIX and I don't think I saw any for Super Mario Galaxy 2, but K&L2 and Mafia II?  Their ads have been everywhere.  Final Fantasy XIII, not quite as much, but still more than anything Nintendo promoted.  The highest promoted thing I've seen from Nintendo is, strangely, Metroid: Other M.  I guess if they throw money behind advertising this game, people will overlook how dreadfully amateurish it is.  Maybe Mafia II is being so heavily advertised to avoid the fact that it's reviewing pretty poorly...

Oh, Sin & Punishment did receive some advertising--but not very much.  More, I think, than Mario Galaxy 2 did.