Makes sense considering the Wii U is a system like the Gamecube where it's mostly appealing to the core Nintendo fanbase. Smash Bros appeals more to the core Nintendo fanbase then any other series, so I'd imagine most of the current Wii U owners would have the most interest in Smash Bros.
Of course it also helps that Mario Kart 8 did increase the userbase for the Wii U in North America by a few hundred thousand. So if everyone who had Mario Kart 8 pre-ordered, also went and pre-ordered Smash Bros, just add some of the new owners since Mario Kart and boom, more pre-orders.
Either way, the Wii U version of Smash Bros is looking to have some good sales in North America this holiday.