wheel torque is less than torque at the flywheel from drivetrain friction losses mainly.
wheel torque being the torque at the wheels....is what the dyno tells you
torque curve falls off because ...
the amount of power..i.e. torque robber in this case goes up basically squared (2x speed = 4x harder)
same with friction... dont want to get into that too much, but oil wedges get smaller and the pressure goes up but it all causes a loss making the engine, numb its ability to accelerate quick on its own not even counting a load on it.
thats just engine related... not counting drivetrain components & mass of everything after the crank-