It's an easy place to carry one that is both easy to reach with either hand as well as doesn't interfere with access to anything else on a duty rig. From a uniformity perspective, if everyone carries a TQ in the same location, that makes buddy aid significantly easier...especially in the high stress world of treating friends and colleagues and time is of the essence.

I've carried TQs on buttstocks (still do on my home defense rifle), as well as on a holster. Carry them where you can get to them and know how to use them correctly, that's all that matters.