Yep. The Jones Act
“Can’t” isn’t true.
It’s like saying “Walmart can’t ship goods between its US warehouses and US stores because foreign truckers aren’t allowed to ship interstate”.
Well, duh.
If the US had an open registry, there would be US-built LNG ships; there's no shortage of competitive American shipbuilders that make vessels small enough to be exempt. But the Jones Act pissed that away, as protectionist laws invariably do.