I'm not sure if you could say miniscue investment without clarifying it (time, money, etc..). For example to support an OS you have to be actively fixing it (the more developers the better). You have to be actively promoting the OS as well (Apple, Android). You have to make connections with developers and other companies to interest them in your platform. And I think probably to the heads of Nokia, they think that given Maemo and Symbian, one which has the proven userbase (Symbian) that they should invest more of their time into Symbian rather than Maemo.
For example, need your device in smartphone mode (e.g. your not doing anything that needs the mobile computer aspect) then just use Android. Then boot into Maemo when you need to edit a document, give a presentation, or whatever the heck you do (or not do necessarily since my use case is probably not prevelant). And you still retain phone call functionality in case someone calls you. Of course the boot switching has to be fast.