As Jeff says there are a lot of US cars available in 1:43. However, most of them. are white-metal models, from the likes of Brookin, Motor City, Conquest etc. Even the cheapest of these retail for £50+. These ranges also tend to concentrate on cars from the 1930s-60s.
As far as die-cast models are concerned, they are out there, but, in nowhere near the numbers of European & Japanese cars.
I think, the reasons for this, are historical. In Europe, Dinky was the first manufacturer to make models, to a scale of 1:43 or thereabouts, pre WW II - they did vary from 1:41 to 1:48. Dinky's model cars were originally intended to complement Hornby's (Dinky's parent company) "O" guage train sets. Corgi followed in the 1950s - I think Corgis used the same scale, so that their models wouldn't look too out of place next to Dinkys. Other manufactures followed suit - Tekno in Denmark, CIJ & Norev in France, Marklin in Germany etc - there were many such makers. Hence, 1:43 has remained the scale of choice since.
In the UK, there was also Matchbox. As they were made to "fit the box", scales varied widely. However, the cars would generally be about 1:60. Matchbox models were widely exported - the wealthy USA being a prime market. Of course, Dinky & Corgi also exported their models, but, it would seem, that Matchbox models were more popular. US firms picked up on this - Hot Wheels & Johnny Lightning - and seem to have settled on 1:64 as their scale of choice. Now, the choice of US cars in 1:64 is widespread.
Quite where 1:18 fitted in, I'm not too sure. Were Bburago the first to mass-produce in 1:18? Anyway, 1:18 picked up a following in Europe, but, it was really embraced in N America. Manufacturers, to state the obvious, are going to make what sells. Some 1:1s are universal - E-type Jags, Beetles & Corvettes for instance. However, for the most part, cars which are unknown in Europe are not going to do very well & the same applies to the N Americam market.
Nowadays, I think, with advent of the internet & sites like this, more people know what is available everywhere. Europeans are buying models of muscle cars & Americans are buying model rally cars....consequently, more US cars are becoming available in 1:43 - Norev, Ixo, Minichamps & Spark are branching out into this area. Of course, in 1:43, US cars are nowhere near as prevalant as European cars - things are improving though.
Any other theories?
:cheers
Steve