I should note that the most powerful of longbows are still fearsome weapons. As Ellri says though, an archer takes a very long time to train, whereas a crossbowman can be brought to a battle effective level of skill in a fraction of the time.

The reason crossbows are considered too much here I think is that its the difference between one or two men per battlefield being able to take out a plate armoured warrior from afar, and every single ranged weapon wielder being able to take the plate armoured guy out.

If that all makes sense.