The title says it all (except I think the word I was looking for was omnidirectional, not mono-directional...oops; I knew it sounded funny to me). Anyway, I know bass is not directional like high frequencies are, but I would like to know why this is the case. What mechanism in the ear/brain causes this to be true? Or maybe it just has something to do with the way the bass couples to the room? Thanks!

Well think about it. A 50 Hz sound wave has a wavelength of approximately 20 feet. The phase difference as it reaches one ear then the other is a small fraction of a cycle. The brain can't discern such small differences.

That makes sense, perfect! Thanks for the quick response. Any idea what the minimum detectable phase difference is? (I've heard the minimum necessary time delay between the left and right ears is 10 microseconds, but that's frequency dependent I assume, with more time being needed for low frequencies, because, like you pointed out, it's really the phase difference that matters...and I assume the minimum phase difference would be constant with respect to frequency, whereas the time delay shouldn't be, I would think.)

IANAX (I Am Not An eXpert). To guess wildly, though, I would hazard a hypothesis that it's the way the bass couples with the room, based on the relatively long wavelengths, but I'm not quite sure.

Also, when someone says "bass" and someone else also says "bass", they could be talking about different frequency ranges. For example, when I press the "bass boost" button on most cheaper portable players, it doesn't boost what I call bass at all, but more like mid-tenor or something, like around 125-250 Hz (or maybe occasionally dipping down into upper bass, like 60-80Hz).
In my opinion, bass really doesn't start until you get down to around 33 Hz, and once you're down to 27Hz or lower (16Hz, anyone?) then it really is bass. Like someone (I don't know who) who has talked about Bosendorfer Imperial (model 290 - 9'6" / 290cm) grand pianos has said ("on the lowest notes you can hear the individual ticks of the vibrations of the note" (or something like that - I'm not 100% sure if I'm quite quoting him/her exactly)), if you can hear the individual ticks of the vibrations in the pitch you're listening to, then it's bass.
Here's a sample of the Sydney Organ's 64' Contra-Trombone stop. A C-major scale (approximately 1 note per second or a little slower) is played using only that stop, starting with the low C on a normal piano (3 octaves below middle C) going down past Bosendorfer's low C (4 octaves below middle C), ending on the organ's lowest note at around 8.176 Hz.
Also, here's a few short sample clips on a baby grand piano including some low (relative to the range of the piano) bass notes.

edit_before_post: in my taking time to get this post (and one of the audio clips) ready.. I see that a few people have already replied before me...