They wanted it to be rack mountable for data centers. Racks are typically 19" wide.
The case is nowhere near 19" wide. They could have made the case 19" so it actually could be rackmounted and would have had enough space to mount a (user supplied) ATX PSU inside AND put the modules in a line.
I never said they were 19". You can put something in a rack that is less than 19" with a shelf but you can't put something in a rack that is over 19" wide which the case probably would have been if they put 4 boards next to each other.
They could have made a 3 module 300GH/s unit for 19: racks with the modules nicely side by side in a good push pull cooling setup.