[quote=Ellri]
those suits of armor are designed for horseback. They don't have armoring between the legs. If you're up on a horse, you don't need armor plating there. But if you're not on a horse (or similar animal) with those suits, you can be sure that the opposition will aim right between the legs.
[/quote]

Also in reference to this, you're referring to Cavelry or Jousting armor, which is only an armor used, as you said, by mounted knights. There were also, however, infantry knights and the king's guard, neither of whomever rode on horseback and so wore armor which covered the inside of their legs, not to mention knee length chain hauberks and a bajilion layers of padding underneath.