Have you flown international business or first class before?
It costs a much larger premium than the space differential, it largely costs more due to price discrimination, because many fliers in business aren't the ones paying for their tickets. If you're talking about real first class (which can carry up to a 30-50x price premium on the economy rate), there's also very attentive service, nice champagne, multi-course meals made by an onboard chef served on china plates with silverware, seclusion, sometimes even private rooms. I would not characterize it as "largely because of its larger legroom".
If you mean to say that many people choose business/first because of the additional space, that's more correct. But even in business, it's more about lying flat and other factors than simple legroom.