If they were truly hiding some secret villain's lair, why wouldn't they cover their tracks by making it look like any other expanse of white snow?
Why not do both
Not really. The reason seems pretty straight forward to me, "The vast majority of Antarctica is also in low resolution due to the bright, often featureless, ice and snow making high-resolution imaging both difficult and largely unnecessary".
That's where they activate the portal duh. Don't tell anyone, lives depend on it, human and non-humanoid.