For many reasons. I happen to really like the sounds you can get from a 12" speaker
on bass. Most of my favorite bassists have played through 12" speaker rigs. I also
like that it's not the 4x10 or 8x10 which is vastly overused in the bass world. I play
a lot of chords so I like the added clarity from guitar cabs.
Currently 12's are very fashionable for bass, using one 12" speaker you can bridge the gap between a 10" and a 15" speaker, generally punchier than a 15, but with more bass response than a 10, which is why they are so popular in a
portable (Portability is obviously not an issue with you since you seem interested in an 8X12 cabinet) 1x12, or 2X12 configuration.
Several high end/boutique manufacturers build bass specific 12 inch cabs; Bergantino, Aguilar, Epifani, Schroeder, AccuGroove, etc., and they sound great for bass. Because they are
bass cabinets loaded with
bass drivers.
The reason 4x10's and 8X10's are "vastly overused" is because they can move a ton of air and still maintain the articulation a 10" driver can provide. It's not a conspiracy, since soon after the introduction of the SVT multiple 10's have been an icon for big bass rigs.
I'm a Sunn aficionado, but for most live applications it's
real hard to beat an SVT with 8X10 cabs.
As far as I know, a bass guitar has a substantially lower register than a guitar, particularly with current 5 and 6 string basses. I guess I just don't understand; If you play a lot of chordal stuff with a bass guitar, why you would expect a guitar tuned cab would provide clarity over a cabinet tuned to cleanly reproduce the frequency your instrument generates?