Why was this filmed overseas ?

Quite a bit more of Jedi was filmed in the USA compared to Empire, likely either to cut costs, stay closer to California, or both. I think Lucas also had either just gotten divorced or was in the process of getting divorced around this time, which may have factored into it. In any case, a lot more of the production was filmed closer to California. Empire went way over budget and Lucas was trying to avoid a repeat.

{JEDI LOCATIONS}:
Death Valley National Park, California, USA.
Buttercup Valley, California, USA.
Yuma, Arizona, USA.
Blue Canyon, Hopi Indian Reservation, Arizona, USA.
Grizzly Creek Redwoods State Park, Carlotta, California, USA.
White Pocket, Vermilion Cliffs National Monument, Arizona, USA.

not shooting it in America takes away from the production quality.

If anything, Jedi looked cheaper than Empire. I'm not exactly sure why, but it did.

Most of ANH was filmed in Tunisia.

Everything on Hoth, Dagobah, Cloud City and the Star Destroyer was shot on location. Not sure about the asteroids tho.

Foreign snow was cheaper than domestic in the 1980’s. It was a purely economic decision.

It was shot in the UK. Travel to Sweden is shorter vs back to the US. It makes sense to shot in the barren snow land of Sweden.

The production quality is just fine. What are you talking about?? It's the best of the bunch

It goes back to Star Wars (1977) which was anticipated by the studio to not be a huge success and was given a relatively low budget for what they wanted to achieve on screen. As a result, the production ended up in England where it was a lot less expensive to make the movie and used their sound stages at Elstree and the crew, and many local actors as per the rules of shooting in another country.

With Star Wars being the massive success it was, and knowing that sequels generally turn over much less box office (though sequels weren't really a thing back then), it was only natural that they turned back to Elstree to make the second movie and duplicate that magic.

Both films also shot on locations which provided fantastic sand dune and snow scenes which helped Lucas give it an "other world" quality which would not have been achievable in the US as many TV shows and other movies had pretty much used many such locations.

OP, comedy genius.