The failure was not due to fatigue, as far as I can tell. Your pictures aren't great, but fatigue on something like this would show some amount of discoloring on the fracture surface. From what I can see, the fracture surface shows only evidence of rupture, i.e. the bolt was overtightened. Rupture surfaces are rough. Fatigue is smoother.
Most of these break due to careless mechanics at work.
I don't have the equipment to build aluminum frames, and I'm not sure I would do it if I did have the equipment. But on the occasions where I'm buying a bike, I buy aluminum frames if it's an option. Steel is a premium option nowadays, so the less expensive steel frames are more expensive than aluminum frames on what is otherwise the same bike. I prefer to pay less for a lighter aluminum frame.