Well, for a couple reasons, control for expansion/contraction (this is when I actually get a chance to meter properly- see my "tri-x expansion" thread), and the "hands-on" feel. I have been careful and done up to 12 sheets with no major sctatch issues. Also control of agitation, stand, semi-stand etc.
I also just got sick of all the tank/Jobo debates and wanted to stick with something I could use as I move up in formats (I keep dreaming of larger negs!) and concentrate on my work.