^^ Maybe from 1970-1980 or whatever, I think there have been plenty of examples of cars not exploding in the movies.
It's a lot more common than it used to be. Films feed on their own conventions and tend to escalate them over time. If you look at
On Her Majesty's Secret Service, the big mountain-road car chase scene in the middle, when the cars crash, they just crash, or maybe catch fire a little. But there's a much later Bond film, either a Dalton or a Brosnan film, where there's a snowmobile chase in which every snowmobile instantly explodes the moment it hits anything, even a bush. Honestly, they go up so easily that it's like you'd have to be careful not to sit down too hard lest you blow yourself up.
EDIT: Besides that, even if not I'm not sure how many people actually believe this happens every time. Is there a contingent of people that actually think a car would blow up every time it goes off a cliff?
Aren't you splitting hairs to a rather extreme degree? It doesn't matter whether some threshold percentage of the population believes it would really happen every single time. The point is testing the myth as it exists in the culture, as it is presented in its source, whether movies or television or folklore or historical accounts or tall tales or whatever. This is what they do. This is what they've always done. They test the myth as it is presented. And this is certainly one of the most pervasive myths of film and television. I don't see what the problem is.
I know the show can't use live pilots/subjects in most of their myths, but it really grates on me how they overlook the major impact a living pilot can have on the success of many of their myths. Apparently if a dead-weight dummy just thrown into a carriage can't do it, it can't be done. Which is silly in and of itself.
They also tend to ignore the tenacity of a dedicated person would have in testing and designing something. Especially something they, themselves, would be getting inside. If it can't be done on your first shoddy try, MYTH BUSTED.
In this case, the occupant would pretty clearly have been killed on the first try, so I'd say there's not much chance he could've tried again.
And what do you mean, "shoddy?" This myth was tested using far more advanced and reliable materials and propellant than would've been available in the 17th century. Hell, they went to
the actual White Sands Missile Base and used real rocketry experts to help them (and how amazingly cool is that?). That meant their version was far more likely to work than something done using period resources would've been. And since it failed spectacularly, that makes it pretty conclusive that it wouldn't have worked with cruder materials and less reliable propellant.
Also, you need to understand that television shows are edited, and that the US versions of
Mythbusters episodes are more severely edited than the full-length Australian ones because of the commercials. The Mythbusters always conduct more trials than are shown on TV, but the editing strips it to the basics.
Either they couldn't afford to do another take or they just didn't care when it blew up so long as it blew up. 
