What you have to understand firstly is that Nintendo mainly sees their software as a way to drive the adoption of their hardware. They want to gain marketshare for third parties to use for their software. The most important thing for them is to get WM+ into as many hands as possible and weighing up the options they have, Wii Sports was obviously the best chance they had to push the hardware.
Of course they aren't going to stop there and as I said, Zelda Wii will most likely require WM+, as will more inovative titles like Art Academy or something in that vein but it's all about creating install base for third parties and so releasing a bunch of first party games all at once isn't the right way to go especially with third party titles launching along side the peripheral. Wii Sports Resort is the first step and expansion into other genre's will follow naturally when Wii Sports Resort is no longer driving the install base as it should be.
