Andrus exists within the cosmos of normal fantasy role playing games. This includes the presence of all the expected inner and outer planes of existence. However, Andrus has a buffer demi-plane between its material form and the outer planes (celestial and infernal). The inner planes (elemental, positive and negative) are a still just a curtain’s breadth in various locations across Andrus.
This demi-plane manifests in the material world as a network monolithic stones that, once properly activated, open into tunnel like passages between Andrus and the outer planes and even between stones. To the uninitiated this network is winding and complex to navigate often leading travelers astray. To those that have studied the gateway stones, navigation can still be perilous as encounters with emissaries from the other planes of existence tend to be hostile.
The Tauran clans successfully navigated passage from their plane of origin through their innate understanding of labyrinths. The Waer spent many years traveling the network of passages till they managed to find entry into Andrus on behalf of the Fae. They managed to find natural elemental breaches through which the Fae could pass into Andrus.
The ancient masters of the human tribes travel between material worlds using gateways linked to celestial events and complex calculations thus their travel to Andrus was unimpeded by this buffer plane. With the World Gate dismantled and celestial alignments being out of sync the masters return is effectively blocked.
This buffer plane may also be a contributing factor to the lack of divine and infernal influences. If an outer planar influence wants to manipulate Andrus they have to send emissaries to establish a foot hold.