Regina believes there’s no way to get a portal back. But that doesn’t make it fact. Regina also didn’t think she could rip Snowing’s heart in half and stick it in two bodies until she did it. And, the portals might not be able to present themselves within the reach of the curse, but as Hook mentioned, he’d been sailing the realms…so other lands may not have the same issues as their kingdom because they weren’t affected by the original curse.