Because a car isn't as valuable tool when it comes to protecting yourself from tyranny (in the NRA's opinion). The NRA's point is that the government should be incapable of immediately identifying which citizens would be capable of forcibly resisting it, for the protection of those citizens who can resist (so that they aren't singled out) and those who can't (so that the government doesn't just pick easy targets).
Yeah, I'm not trying to point out hypocrisy or anything silly like that. Still, it does seem at odds. Certainly, if I were running an oppressive government in the US, the list of NRA members would be top priority.
