Why is the injection pump speed is twice less than the crankshaft?

Answer 1

I think you mean half the crankshaft speed.Its because its a four stroke engine,the piston only fires every second time it come up.

Answer 2

The whole engine cycle is done by two rotations of the crankshaft

A fuel injection is required once in this cycle

So instead of it going the same speed as crankshaft (where it would inject twice) the pump would need to be slowed down to twice the speed to ensure an injection once every two rotations of the crankshaft.

A bit weird to explain but just try some sort of physical demo with gears or something :-)