Brewing thermostat

I've no idea specifically why it didn't work, but generally I can say that my experience is that if all connections are fine (triple checked and measured), you should have a look at the power supply. It is one of the things that also changes when moving off from the breadboard and into "reality". Powering from an USB, or desktop power supply with nice smooth voltage and steady current is something completely different than a dodgy old car battery, solar cell or whatever you end up connecting you thing to in the end.