I think that was because the trials and dungeons were sort of made for the Relic quest (mostly Chimera and Hydra) and they were fun. Everything after that, though... nothing 'new' (aside from the Mysterious Map mobs, that was a step in the right direction) was made for the Zodiac quest line. You could argue the Nexus is the Relic quest all over again, but with more freedom(?) :/