Before a portal connecting two worlds can be created, someone has to travel to the planet and set up a physical gate. Settlers have to travel through the astral plane with ether powered ships capable of jumping being the worlds. These jumps can only occur when ley lines connecting the planets throughout the universe line up properly.
The ley lines shift over time and the further apart two planets are, the longer it can take for them to align properly for a jump. Planets close to Nexus might take only ten years but further away planets can take thousands of years. Fortunately, the ley line alignments generally follows a cyclic pattern and mages who study the astral plane are able to predict their movements.
Once settlers jump to a new planet, it can often take a year or more to setup a paired portal. In that time they are cut off from Nexus and unable to jump back. This makes astral travel dangerous, but the rewards for opening a new world to commerce are so great that many adventurers continue to risk themselves.