Apart from the lack of political interest, there's obviously a "cultural" barrier which tends to push the average Americans toward less-efficient vehicles. I don't hate full-size trucks or SUVs, actually I like them a lot, but sure they're not as efficient as they COULD be and it's more a matter of that mentality which tends to associate the power and the amount of cylinders as how manly a vehicle is. If a soccer-mom wants a Tahoe, for example, I don't understand why not to get a 4.3L V6 into that (apart from the absence of this option for the Tahoe), or even some 4-banger, eventually turbocharged such as the 2.0L Ecotec.
It was discussed before, but the perception of the amount of cylinders as a premium feature is one of the reasons why American vehicles often don't get the best efficiency they could. For example, many upscale European sedans actually are also offered with 4-bangers while a similar Cadillac wouldn't have anything smaller than a V6. Not just the displacement, but also the lower amount of internal frictions and the less restrictive intake and exhaust setups also help to balance efficiency and performance.
I've already seen many older American full-size sedans and trucks (including SUVs) which had its stock engines, either straight-6 above 3.0L or V8 often bigger than 5.0L, replaced by smaller 4cyl Diesels between 2.2L to 3.0L, and a handful of them even with 2.0L 4-banger gassers. Also, different gearings (usually with 5-speed stickshifts replacing older 3 or 4-speed ones) helped to not make it feel so underpowered, but couldn't make miracles
