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.
As for why they don't (release Android phones). I don't know. I don't even think there is a stringent licensing requirement from Google (unlike Microsoft's Windows 7 Phone series). Unless you have to pay Google to use their app store? Or maybe they're just afraid they will wind up as an Android hardware maker.
Maybe if you consider Maemo as a smartphone OS.
I think Maemo could supplement Android by providing the mobile computer side of things. 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.