Because not every one uses back heat. With belly heat the probe either needs to be in the tub or on the tape its self. There is not many other options, if you suspend the probe from the top the bottom will exceed safe limits. No two set ups are the same. I thought about back heat but in my conditions with the short test I ran I could not get the temps past 83 with the flex watt plugged into the wall. I use interior mounted probes for static enclosures but a tubs in a rack there is simply too much opportunity for disaster. Probes must be firmly fixed where ever they are and in a moving tub has too much chance for failure. There are many ways to set up a rack the one that keeps correct temps 80º cool and 90º warm is correct. Back belly or overhead it makes no difference as long as the temps are right.