Ah, yes, hygiene. Truly a fate worse than... uh...
Both cats and rabbits hate being bathed for nearly the same reason:
Their coats are adapted to repel water, but traps it if enough water gets on them. This hinders their grooming, makes them heavy, uncomfortable, and, worst of all, cold. So, both creatures tend to avoid water if they can.
Additionally, hygiene/grooming is really important for cats and rabbits as a way to show dominance and subservience, respectively. Higher-ranking cats groom others to show dominance and dominant rabbits tell the others when to groom them.