Because SIG Sauer GmbH in Germany/Switzerland and SIG Sauer Inc. in the US are two completely separate companies. SIG Sauer GmbH, the company behind the 751, does not have a presence here.
Seeing as how the firearms business has razor-thin margins as it is, creating a presence here to manufacture the rifle (which is not importable) would cost more than it'd be worth I'd think.
Maybe SIG Sauer Inc. will grace us with a 7.62 variant of the 556 one day. There's the 716 they promised back in January, but that's an AR10-patterned rifle, not a 750-patterned rifle.
-T.