Because compared to a
monohull, cats have higher cabins, large biminis,
davits with
dinghy, and
solar panels: there just isn’t clear air back there for a
wind vane to
work on a cruising cat.
Possibly with one on each stern, using only the windward one?
Next problem is to connect to the
steering system, which is generally much more complicated with two rudders and their interconnected
cables, chains or hydraulics. And often much further distance to the wheel(s).
Biggest issue might be the greater range of apparent
wind angles for a given true
wind direction because of a greater range of
boat speeds. A
wind vane would be
steering all over the place in many reaching conditions.