Because its owner is a 6' 2" 18 stone ex-Cumberland and Westmoreland wrestler, and if he wants his brake on the back, no-one's going to tell him no.
It is the original seat-pin, as they used to be called. It goes round corners as if on rails but is strangely responsive for such shallow angles . Wheels are the same size, he has a pair of Constrictor "Circumferential" wheels hidden in his house, the spokes instead of an elbow, go round circular bosses on the hub flange and back to the rim, 16/18 gauge spokes and wooden rims.