Just a thing that pop into my mind after reading some post in this (and several similar) thread...
Could it be, that it's quite hard to make a superb track when the physics aren't "done" yet? Some Finn said earlier in this thread, that Aston's eau rougie feels weird. It's just that we or eric can't actually know how it will feel after next physics update.
I would presume that building a track takes a lot of test driving. Same goes with new cars. No point in pushing more tracks and cars into public when next physics update could change the balance (or idea behind track and car) and therefore produce much more tweaking for the modeller. Easier to keep tracks and cars "half-done" and finish them little by little.