Because it's fun.

To be fair, 250W on an ebike is 'not fun' or anywhere close to the legal limits in the US (750-1000W).

The happy spot seems to be 2500-5000W for most hub driven ebikes. I turned my bike down to 1000W this afternoon and couldn't imagine running anything less.

Endless-sphere [1] is the place to go if you're interested in eskateboards/ebikes with a bent towards DIY/highpower.

[1] http://endless-sphere.com/forums/viewforum.php?f=35

Skateboards apparently have a much higher rolling resistance than bicycles: http://books.google.be/books?id=0JJo6DlF9iMC&lpg=PA162&ots=T...

The power multiple between a Boosted board and others' technology is the same as between a Tesla Model S and a Nissan Leaf. Both cars can go at highway speeds. The extra power of a Boosted board is important for practical reasons, like hill-climbing and braking, and for fun.

I can only speculate, but perhaps it has to do with the regenerative braking the boards provide. On an e-bike you still have friction brakes, but on one of these boards you need to be able to brake quickly using the electric motors.

More powerful motors, while perhaps not strictly necessary, would also provide quicker acceleration. For a product that's designed to be fun, this might matter more, vs e-bikes which are mostly designed to be primarily utilitarian.