Simple answer, especially when retrofitting a facility. Cost. I've been down this road. I had to reject 3.5MW of heat from one part of a facility, while the rest was office space which needed to be heated/cooled. I would have had to do a serious retrofit to the existing HVAC system in order to reuse that heat. It was a nice-to-have. When costs run over budget, and Value Engineering kicks in, nice-to-haves are the first thing to go. I wish that it was as simple as piping in a plain old Heat Exchanger, but that doesn't always work.