Simple answer: Because in the NBA, defense is almost an afterthought and something only occasionally employed by championship teams (and only a rare few of those). See, e.g., James Hardin
More complex answer: Because guards are shorter and thus largely confined to shooting outside the arc, and the big men down low often have their backs turned to the basket and thus can't see the entire court, it is natural for the guards to run the offense.
Even more complex answer: In basketball generally, unless a team is desperate in the final minutes, the bringing of the ball over the center line is almost never contested.
Most complex answer: Outside of power point goals, probably most goals in hockey are scored on odd-man advantage rushes, and for that "the offense" shouldn't consist of a defenseman at all unless they were horribly out of position to begin with.
Point to ponder: What hockey would be like if there was a 24-second shot clock.