Max pwr take off
 

Why does the ASI flicker during max pwr t/o ?

One reason is the acceleration is a little faster the heptr will pass through the ground vortex around which causes instability of airflow.

Is there any other reason especially due to the construction os the ASI ?

You basically explained it. Until the pitot is clear if the down wash, it's not getting clean air to be able to produce the needed ram-air effect of forward movement.

The down wash can also create a vacuum effect inside the pitot. It's not uncommon in certain helicopters with glass panel avionics to show a negative airspeed until you've past translation.