If you play a high-frequency sound from one speaker, it sounds to your ears like it is coming from the direction of that speaker.  Lower sounds are harder to locate with your ears. (Ever had a weird humming noise in the house and spent hours trying to figure out where it's coming from?)

I think really high-pitched sounds are hard to locate too, so really our ears are best at locating midrange sounds.

This is one of the reasons baselines are usually mostly mono.

This makes a lot of sense. I never knew about this for some reason until both you guys mentioned it. It's a pretty useful bit of information. I'll remember as I'm making my mixes.