This is addressed in the canon novel Aftermath: Empire's End. Routes into the 'Unknown Regions' are basically impassable.
Above the computers is the projection of a three-dimensional star chart that matches no known map here in the galaxy. Which makes sense given that it does not chart the known galaxy, does it?
For decades, these computers have been plotting a journey. Outside the
known galaxy is an unexplored infinity, Palpatine explained, one
closed off by a labyrinth of solar storms, rogue magnetospheres, black
holes, gravity wells, and things far stranger. Any who tried to
conquer that maze did not survive. The ships were obliterated, or
returned to the galaxy devoid of travelers. Communications from those
explorers were incomprehensible, either shot through with such static
as to make the content useless, or filled with enough inane babble to
serve as a perfectly clear sign that the explorer had gone utterly mad
out there in isolation.
As to why this area of the galaxy hasn't been explored more closely historically, the short answer is that certain routes have been identified over thousands of years (to Ilum, for example, by the Jedi) but that the cost of sending hundreds of thousands of probe ships to their destruction in order to map the very small number of hyperspace nodes that'll give you access to this region simply isn't something that anyone contemplated until Palpatine decided it was a decent way to spend the Empire's money.
The computers here have long been searching for a way through the storms and the black spaces. Slowly, surely, they have been putting together a map: a journey into chaos. The Empire has sent probe droids to test the computations as the computers have made them. Many never returned.
But some kept reporting in, pinging the transponder here. Every droid that made it further contributed to the map. And with distance achieved, the computers, through the scanning droids, continued to chart the course and compute the next branches of navigation.
