Confused with mesh analysis
It's because of the current source and the fact that I2 is the only mesh current in that branch. I2 can't be anything other than -2 amps, so you don't need to write a loop equation for it.

If you swap the current source and the 12 ohm resistor, then you get an extra equation (I2 - I1 = 2 amps) and an extra unknown (the voltage across the current source).