This is more the fault of the retail chains, and the deals made
I remember, when XBOX came out, Microsoft paid big money for promotion and support from Coles Myer - who own Myer, Target and K-Mart, probably the 3 largest department stores for games buyers. Consequently, ever since, XBOX has had special displays, whilst the GC gets backed off into a corner. Likewise with catalogues - they've always featured PS2 and XBOX games, but GC games have always rarely been advertised.
Plus it doesn't help that Nintendo doesn't do industry nights. Both MS and Sony have nights where staff from these dept stores go and get the hard sell on their respective upcoming products. I've seen staff coming back with absolute biased opinions on consoles, and this directly affects sales. (having the person selling consoles, continually going on that "simpsons:road rage is sooo much better on xbox than GC, because GC is a kiddy console" is proof positive of that)
So yes, this is mainly a problem caused by the ever lovable (not) Coles Myer. However, it's one that Nintendo should be actively trying to reverse, and why they haven't is completely beyond me. Some more advertising wouldn't go astray either.