One reason may be for the unbalanced gear that is out there...so economics being what they are, one less wire, one less connection on the bay...etc. Also, most home/consumer gear is unbalanced, though I don't know who ever needed/used a bay in their home stereo setup, but I have seen a lot of home/consumer gear end up in pro audio setups.
There's nothing wrong with un balanced gear...or unbalanced wiring and connections...they just don't handle noise issues well, but if you wire everything right, and keep it fairly short distance...unbalanced works perfectly fine.
Way back in the day...I had some 1/4" TEAC/TASCAM unbalanced bays.
These days, I use only balanced bays, and for my unbalanced gear, I just wire it accordingly so it can work alongside the balanced stuff. Never been a problem.