My understanding was that Cosco, the shipping company (you've probably seen their containers on trains or trucks), wouldn't carry them anymore because their ships stop in Seattle before they get to Canada and the US has a ban on Chinese ammo and firearms. Because of this port authority/homeland security/customs/some-one-else was giving them grief for carrying items prohibited in the US even though they weren't being dropped there. It wasn't worth the hassle since the items in question made up such a tiny portion of their cargo so they stopped allowing it.
AFAIK we can still get ammo from Europe so the ex-Soviet surplus is still okay to come in; just not anything from China. As a result that means no more 5.56/223 or 7.62/308; only 7.62x39, 7.62x54R, and 5.45x39.
I could be wrong since I wasn't involved in the thread with dozens of pages talking about it but did read through most of it.