It has nothing to do with what Live For Speed is or isn't, but what the 'rival' sims
are.
Let's take Grand Prix Legends for example, the vast majority of people who play it are those who grew up a lot closer to the 60's and have a stronger bond with the cars and tracks than is probably healthy.
The same can be said of GTR, endurance racing for the most part doesn't appeal to the younger audience. Setting aside 3 hours (at least) to watch a motor race doesn't fit in with their plans like the way touring cars and Formula One do.
Excepting the LX cars, the other deciding factor is that Live For Speed doesn't really model old cars. It's all relatively modern which probably alienates some of the GPL purists who might otherwise enjoy the physics and online racing that LFS has to offer.
Above all though, Live For Speed manages to offer everything from Mini sprint racing to full grands prix, which is remarkable, really. So I guess the younger audience is downto maybe being more open to what's on offer and not being turned off by the fact it doesn't actually simulate a real series or track.
This long ramble was brought to you by the letter C.