I believe a non-US company can get FCC approval (many of the phones that never officially make it to the US from non-US manufacturers have FCC approval stickers). But a large manufacturer/re-brander with a strong US presence will have an easier time selling carriers on a phone than a company like AnexTek, and can offer the level of support carriers require.
My two cents:
1) Carriers are still the biggest retail outlet for phones. The economy going south seems to have scared them away from high end Pocket PC phones-- that's why the XDA II isn't sold here *sigh* Though these phones are often purchased by power users who crave high end features.
2) Samsung is making some really, really lovely Pocket PC phones for Korea, but they haven't made it here yet. Verizon has a strong agreement with Samsung, so they'll get these phones before Sprint. However, Verizon takes eons to test and qualify phones *grr*
3) There are some seemingly really nice overseas Asian Pocket PC phones and some low end ones. The Audiovox PPC4100 and Eten P300 are examples of the low end phones. Carriers think budget is the way to go right now, but sometimes these budget phones have more problems or need more tweaking before they come to the US.
I think the PPC4100 is pretty solid, esp. with GPRS working in the US (I've heard the original AnexTek has problems with GRPS in the US). But it still seems to not have great RF, has low speaker volume and the first ones out the gate from AT&T are supposedly prone to hangs.
I've got an Eten P300B for review right now, and it will be marketed in the US soon by FutureComGlobal. It definitely needs some tweaking before a US carrier would touch it! *ouch*
Editor in Chief, MobileTechReview