An interesting question. Perhaps because for their purpose it does not work. They don't want the same things from their MA that an aikidoka is looking for from theirs.
Take this with a grain of salt this is simply my own observation based on my limited experience.
Lots of people take up a martial art because they want to be able to hurt someone and or win a fight. Their minds are on preparing for a fight and they are looking for the brawny way out of it. They want to look cool, be perceived as powerful and strong physically, be feared for the damage they can do to someone who might attack them.
Now I understand that in MA of any type, many practitioners ,when they reach a certain level no longer tend to have those desires. But I suspect that the majority of those ,ridiculing Aikido as ineffective, have not reached that level.
Aikido looks weak because in most cases effective aikido prevents the physical fight from ever happening. And should that conflict become physical it still looks for the least damaging way to resolve the conflict. It seems contrary to what most people expect from a martial art. But because of those very facts Aikido is quite powerful. IMHO.
Of course if gentleness fails Aikido has a few surprises in store for those who might have doubts. Let it remain our little secret.
