It depends on the type of Nintendo game.
If it's a game that is worthy of its prior legacy on older systems - like Galaxy - you'll see the support coming out.
The reason being, I think, is that among all the fanbases I think you'll find a heavy presence of 'Nintendo' in the early years of gaming. You might be a Sony or a MS fan today, but there's a fairly good chance Nintendo was top dog for you way back when (it was likely either them or Sega). So when Nintendo comes out with something that these folks feel is worthy of the Nintendo they cherished in their younger years - particularly in the two top Nintendo franchises, Mario & Zelda - the love overfloweth.
That's my theory on it anyway.
edit - the kind of split above in Durante's post would help too
But I think for those kinds of games the support goes beyond self-identified Nintendo fans for the reasons above.