I am not sure if there is a "strong" reason. Nintendo like to promote themselves as a caring, family company - and would like families to be able to invest in them I suspect.
It would also help build (further) brand loyalty, and may even drive sales higher - if I owned a bunch of Ninty shares, I would be a lot less worried about spending even more money on games.
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I'm hopeful Nintendo have something awesome in the wings, that none of us are expecting. I wonder if it would be viable for MS to release something like the MS Surface "as a console" (yes, at $5k/pop). I can see a lot of hard-core gamers purchasing one - I would be a lot more excited about something like that, than say spending 1/10th the money on a PS3.