This is the known-quantity release vs. unknown-quantity release affect.
People knew Mario Kart (and Smash) were inevitabilities as well as Mario. That's why these sorts of titles don't usually see large or long lasting bumps (we'll see the same with the PS4 eventually, and its why the "wait for X" or chalkboard discussions are jokes).
Its the weird, out of nowhere titles that spur on actual hardware interest outside of the bare baseline. Huge IPs bring in spikes, but people know those IPs are eventually coming, so affects of this variety are spread far and thin. New titles and new IP can't have been anticipated (for obvious reasons) and so see their affects take hold after their release rather than before, and look much more "effective". Now, generally, these sorts of affects don't tend to last two months... but squid.