The beach assault was chosen for 2 reasons.

1 The chaos forces did not expect an assault
to come from such a unusual area.
Sea bourne assault are notoriacly slow and there
for to defend against.

2 Flying over the mainland is treacherous due to
an unknown entity,its like a mini warpstorm or psy
protection.

The Inquisition are most interested.
Marine scouts have been in. And from their intel.
There seems to be 3 large structures hiden inside
this area.They seem to be triangulated.
These are though to be generating this effect.

This triangle may have to be broken?