That's because the Lexus board relies on a magnetic rail buried underground to do the levitation. Start watching at 1 minute in:
While it's obviously awesome that Lexus has made a working prototype, this technology has actually been around for years. It's what powers maglev trains. Lexus just shrunk it down to the size of a skateboard and buried the rail.
To mix cultural metaphors, this is like pulling back the curtain in the Wizard of Oz. It's a little disappointing given all the gee-whiz claims about hovering.
Let's hope some other genius floats a better idea.