Why is the planned Kingdom Tower Skydeck unsuitable for use as a helipad?
A quote from The Guardian:


  A Starship Enterprise-like circular helipad, 30m across, protrudes out at level 157. This was originally designed to provide fast access to the penthouse, but the wind makes it a dangerous place to land, so instead it has become an observation deck – the world’s highest at 630m.


link (emphasis by me)

Helicopter prefers steady wind condition in order to land safely and comfortably. The nature of wind at such height is gusty and turbulent due to proximity to the building itself. It would be unsafe and very uncomfortable to the rich man who rides the chopper for 'quick access' to the penthouse on that floor. The highest helipad now is at 438 meters high at that is at the TOP of the building.