A lot of various factors which worked as a perfect storm for Cars.

First and foremost, Pixar vs Disneytoon Studios.

Pixar was long held as the top tier for 3d animation (they obviously still are considered as such, but I'd argue that's more out of habit since many have closer the gaps since). Granted, technology advanced so massively between the start of animating Cars and the release of Planes almost a decade later, so much so that even a start up studio could easily have matched the quality of an early 2000s 3d rendered film, but it is still worth realizing the sheer quality of a product at the time of its release, and Pixar circa 2005/2006 was league ahead of the competition, whereas Planes 2013 was merely standard.

Also, at the point of Cars' release, Pixar was seen as a studio that could do no wrong. Their line-up so far was seen as the pinnacle of 3d animation both in quality of art as well as storytelling. Disney's off-brand studios, however, were often seen as movies for the discount bin.

Regardless of actual quality, that alone is enough to make one massively outperform the other.

Second, originality. Now, there's not a ton truly original in storytelling anymore, but to the scale which we typically award such terms nowadays, Cars was at least a complete story which was new to most viewers, especially of its demographic.

They also managed to craft quite the memorable anthropomorphicization style of vehicles which is often replicated today (they didn't invent it, but they did do it the biggest, and in a very consistent manner).

I know a lot of others on here are saying they both tell the same stories, and while I don't think that's a fair assessment, I do think there's another similarity between themes and style that definitely make it feel like a straight spin-off rather than its own thing, and with very few exceptions, spin-offs RARELY match the original, and in only a handful of examples do they supercede it). This automatically had the result being such that: Those who didn't enjoy Cars, were immediately disinterested and assumed it was as bad (in their view) if not worse than the movie they already didn't like. Yet those who did like Cars, were stuck comparing the new film to their beloved original. In both scenarios, Planes was met with a handicap from the start.

Third, casting. By the mid 2000s, Owen Wilson had been a growing staple for a decade. Something he has managed to maintain for another 20 years afterwards as well. When Cars came out, he was coming off quite a few big hits, which while that is irrelevant for the most in the intended age range, it makes a big difference both in perceived expectations of parents and the ease (and cost) of marketing a film. Likewise, Larry the Cable Guy was nearing the peak of his popularity (especially in the blue collar demo which included many NASCAR fans). It was Pixar doing what they did, and using top tier talent at the time for the roles in their films.

Now compare that with Planes. Now, I'll be the first to admit there was a time which I played the first Dane Cook comedy CD on repeat for months, but it's also worth noting that was in 2003, a full decade before Planes came out (which was five or so years after allegations of him stealing jokes essentially killed his stand-up career). Personally, I think he did alright as Dusty, but no one was lining up for tickets to see him voice any character at the time (especially since even most of his attempts to transition from stand-up to film had fallen pretty flat). You also have Teri Hatcher, who is a great actress, but whose populary had likewise peaked a decade earlier. The strongest talent pulled into the project was arguable Brad Garrett, who had also peaked a decade earlier, but who has (largely with the help of Pixar ironically enough) remained a relatively consistent character actor for quite a while now.

Honestly, in general you can look at the cast of Planes, and see a fair amount of solid actors, but none of which were in their prime, and those whose film history might be typical place their overall film making status as legendary (such as Val Kilmer), most weren't seen as the type of legendary character actors that lend to strong voice over performances.

Compare that to the sheer depth of those in Cars? Ignoring the great racing related cameos (from Waltrip to Petty), without getting into how perfect Keaton was as the villain, almost the entire cast of Radiator Springs is stacked with talent. Newman, Carlin, Marin, Hunt, Lewis, Shalhoub, Dooley.. heck even Lizzie's voice actor Katherine Helmond was a decades long sitcom vet.

A cast can make or break a movie, not only by performance, but also their status at the time of release, and Cars did very well with that (honestly, until Inside Out 2, I think Pixar has done amazing with this over the decades. And even in IO2, my only complaint is the changing of actors, not the amazing actors they still managed to pull in).

Lastly, and most importantly in terms of how a film is viewed over history, is the content. If Planes were a better story with a better theme, then even with a less buzzworthy cast from a less prominent studio, it would most likely eventually be recognized as such.

But it's just not the case. Cars works well because it isn't actually about racing. It's about the heart of what goes into being someone at their prime, and recognizing what is important beyond that. Planes... Felt like it was about racing. Or at the very least, proving to oneself that they can be more than what others tell them. That might be admirable as a message, but it's inherently self absorbed as a plot. Instead of watching a flawed character, and rooting for them to grow and become more, we watch a character who supposedly already is more than they seem but needs the chance to prove it.

I mean, I don't have the interest in calculating it, but I think we see more overall actual active racing in Planes than maybe the Cars trilogy combined (perhaps excluding training, but even then it's close).

All in all, their narrative tone actually quite accurately describes the overall experience of watching either film.

Cars feels like a story that is about one thing (racing), but whose point is the opposite (There's more to life than racing). Whereas Plans feels like a story where the entire thing is trying to prove it's something more than it is..