I think the biggest problem is noise. Large scale is loud. If you have a track generally the county requires a certain decibel to be compliant. At least that is how it is with Air planes and air fields. From what I have read and experienced at my local air field there are certain individuals in the community or county board that try find any reason close down fields that dont comply to the decibel that was set.
I think what is more important is to just to find an accessible place just to bash 1/5th first. Racing comes after you have established locations to bash 1/5th.
Like when it comes to airplanes... it is so convenient... I have a field 5min from and another 20min and another 40 min away. All catering to diff type of planes, some have runways, and some dont... But if I want to play 1/5th... I have to go to PA. No thanks.
Honestly for 1/5th scale to succeed in racing I think it has to go strictly electric (here in the states). Or strict muffler regulation (must have silencer) for gas.
It seems it was hard enough for some airplane clubs to get county help... And airplanes require knowledge, experience, and mathematic (when you build your own plane from scratch)...
What do you really build on 1/5th scale cars?
Making planes is much easier/accessible considering you use balsa and ply while cars need metal, welding, and so on.