Feasibility is the major contributing factor. Time, Space and Money are hard to come by. Then of course there is the red tape from the respective government involving permission for a very costly hobby. Then, the easiest to acquire, knowledge.
"We fly our rockets from the military firing practice area ES D 139 in the Baltic Sea, 20 km east of of the Danish island of Bornholm. It spans 70×35 km, and are opened to us by the Royal Danish Navy for the launch time window. The Danish and Swedish authorities are very helpful, and close the airspace above for airtraffic in the hours of the actual launch."
Also regulation I’m not entirely sure how many countries would let you build what amounts to at least an intermediate range ballistic missile in the back yard.
I’m actually wondering how did they get by with the multiple missile technology non proliferation treaties out there.
The majority of ITAR restrictions (and other similar export laws, in my experience) are for technologies that help with navigation and targetting, not propulsion. The technology to build an amateur rocket is a lot more widespread and easier to build locally or import.
Besides, export restrictions are rarely absolute - they mostly just add paperwork to legally certify that neither you nor your clients are sending the technology in question to Iran or NK.
Try building an ICBM in your back yard and wait to see how long it will take for some government agency to come knocking.
And you don’t need to import it, making these publicly and not to mention possibly opening the plans to the public will trigger some alarms some where.
Even if you take national security concerns out of the equation there are a lot of other safety concerns that would get someone interested.
I personally would prefer that my naighbour won’t be assembling a liquid fueled rocket in their garage :)