Very few of us on this entire site actually own and shoot SML rifles.
However, the OP asked why more are not making SML rifles...………………. Some of the responses are his answer.
You screw up with this ship, your course will be straight to the bottom of the pond.
This is just my opinion concerning SML...…….. I do not like reading posts where anyone, regardless of who, cobbles up something and presents it to an audience. Not even the BEST SML builders will release or show anything, until its actually proven with top quality methods, such as pressure checks, traces, etc. Tying something to a bench, standing behind a tree with a long string on the trigger, is not a valid test.
I'm not trying to belittle anyone or any individual, but myself and everyone must take SML very serious. SML requires 100% concentration and attention to details to the finest.
IMO, let the known high quality and trusted builders work inside and outside the box. When they identify a new mouse trap and have tested and confirmed it, let them release it. If you have an idea, present it to one of the professionals.
It doesn't matter who you are, what profession, what experience you might have, things can and do go wrong. Example: My first muzzleloader came in 1973 and have been shooting them a LONG time. It took me 42 years before my first dry ball......... but it happened. With SML, I've already made one boo boo, but that's why we use witness marks right?
SML requires 100% concentration and attention to details to the finest.