Because a Helicopter commonly operates at speeds <35 Knots. The Pseudo Weight on Wheels (WOW) GPS Derived Squat Switch Input cannot be utilized to determine the Aircraft is on the ground. 

On a helicopter, the use of either a Squat Switch, Collective Switch or Airspeed Switch is required. For Helicopter Installations of the IFD5XX/4XX the on board switch needs to be wired to both the on-board transponder Squat Switch Input and the IFD Altitude D4 input on P1001, pin 70. As indicated, the logic state required is "low when ground". 

This will allow both the IFD5XX/4XX Navigator and On-Board Transponder (AXP322 in this example) to receive a "GROUND" signal from the on board squat, airspeed or collective switch.

The IFD Main System Config Page must be set to:

Airframe: Helicopter

WOW Input (Helo): Yes