When a car which can move at a speed of 100kmph. why cant the same car move in reverse at the same 100kmph?

In Forward motion the gears when comes contact speed of car increases where as torque decreases but when car moves in reverse direction torque is high.

This is because at top gears larger gear is driving smaller gear.

In reverse gear smaller gear driving larger gear. 

If you're at km/h in the4th gear, the gears in the gearbox are spinning at an incredibly high rate. The fourth gear usually means the engine's RPM and the wheel's RPM is the same, (gears1-3 are under drive and gears5-6 are over drive). Which means, the car's engine is spinning at at least RPM (this figure is different for different cars). While the reverse gear is not engaged, it is stationary.  So if you try forcing the gear stick towards the reverse at km/h, it will most likely not slot into place while making a grinding noise. Therefore you will not be able to put it in reverse gear. Also, the vehicle may vibrate abnormally. 

Needless to say, this is bad for your vehicle and should not be done..

it is because of the transmission gear ratio

Reverse gear is similar to first gear in reverse spin. The engine speed to reduced in the process.