Because it's redundant to prohibit things that are already prohibited. I know it goes against VC-ist dogma but, as a general rule, lane-sharing is prohibited in all 50 states (with a few, explicitly stated, exceptions, such as two bicycles in the same lane)-- even in California where the CHP has said that it doesn't care what the law is, it isn't going to enforce a ban on lane-sharing unless it feels like it.
I completely disagree. Why wouldn't all slow moving vehicle laws simply say that the slow moving vehicle must use the rightmost lane available for through traffic? What's the purpose of the "as far right as practicable" language if not to allow for lane sharing?