Whoever was in charge of filling the Unknown Regions with content for the EU really wanted it it be an inhospitable place;
http://starwars.wikia.com/wiki/Unknown_Regions
Having skimmed through the article, it seems like everything there wants to kill you.
However, even the article seems to take a stab at the idea of it being so close to the Core.
"The depiction of the Unknown Regions varies from source to source. Some fans would like to limit the term to areas outside the plane of the galactic disk. It is supported by movie canon, which describes a Galactic Empire, and in which the entire galaxy can be surveyed for any lost planet, fugitive, or Rebel base."
Even the people who’s added to the EU seems to have been confused by it’s placement, even going so far as to make up some excuse about a mass of dark matter in the region that creates “gravitational anomalies that hindered navigation.” The article also talks about "hyperspatial gravitic ripples" and the "intentional construction of a barrier, possibly by the Celestials to hedge the expanding Infinite Empire (That would be the Rakatans) or to contain Mnggal-Mnggal" (the weird creature that darklordoftech mentioned earlier).
Translation; nobody knows why the heck the guy who drew the map drew it the way he did.
(Some of these explanations sound like something straight out of Star Trek.)