FFG did not anticipate the game being as hot as it has been. They printed their first major print run in October, and have probably done five smaller print runs since then. When working with factories in China, it's not uncommon to have to book time six months in advance. So, these smaller print runs have been fit in either by cancellations, or by some buffer time built in from the factories for special prints.
The most recent print run went into stores last week, and has already sold out. I'm not sure if Amazon got any, or if it was just through FLGS channels. I'd imagine that core sets should start to be pretty plentiful around the time Carcosa comes out (purely speculation), which should be around August/September. There will probably be several small prints/shipments between now and then though.
Keep an eye on this thread, and either I or someone else will probably post up if we see if it in stock somewhere.