I think when it comes to the S7 and the SSC its a combination of things: theres not much desire on TGUK's part to test superwide American goliath cars on UK roads so it would have to be a test in America or on the track, either of which requires a lot of coordination and effort; with Clarkson's obsession with "handling" theres not much desire by the S7 and SSC people to have Clarkson bad mouth their cars for not handling "correctly". I've seen footage of the SSC out on the road and it can take a corner quickly, but Clarkson's definition of "handling" seems to change with every car he tests. He would probably start mouthing about how its too stiff or doesn't blablablabla. Either the S7 or SSC would be a beast in a drag race, but they've already had endless drag races with supercars; to the average viewer (ie, 340m out of the 350m) who knows nothing about how much even 200hp feels in a drag race, all of that is meaningless. So unless theres a specific challenge they can dream up, or an "American Supercar Special" (dear god, Please make that happen!), I don't think theres much chance of seeing these cars in the show.
Overall though, I don't think its that Manufacturers refuse to lend Top Gear their cars (though some do), its just that some will help coordinate (Bugatti, Koenisseggeeggessiissgg) and some will actually drop them off (Ferrari, Lamborghini, Aston Martin, Jaguar). If a manufacturer doesn't put forward the effort to be on the show, then the effort (and expense) has to come from TG itself. If they have enough material for a series, why put forward the effort?