Limited demand. They'd be selling a product that's both low quality and illegal to own. The target market for that would be criminals with money to spend, who don't already have access to equal or better guns. And it's not like you can set up shop on a street corner; secrecy is expensive.
Basically, a black market Sten Gun factory has all the drawbacks of illegal arms dealing AND startup costs to boot. I'm not surprised it isn't a thing yet. Oh, and I doubt 3D printing will make it a reality any time soon - the startup costs would be lower, but the 3D printed guns would be even worse than cheap locally manufactured metal guns.
There are places in the world where the locals make cheap metal guns en masse, but they tend to be places like Chechnya rather than the UK.