crossbows don't even enter into it.
Remember, this thread is about fantasy RPG design and firearms, so real-life arguments hold little water.
Even so, big crossbows were typically built to be cocked using a combination of body weight and leg strength.
My question is: Why would you create that first generation of guns that cost more than a magical or mundane equivalent, were no mechanically better, had significant drawbacks or flaws etc?
Because if you introduce the first generation of gun to D&D, it looks horribly bad compared to a heavy crossbow, a wand of magic missiles, necklace of fireballs or anything else that the game already features. Add to this the fact that there are already alchemical devices and magic spells that cause things to catch fire and you're just creating an expensive way to commit suicide...
The other factor to consider is that fantasy RPGs are almost universally set in a state of technological stagnation. In almost any fantasy setting, if you get sent off to a quest to recover the sword of an ancient hero, it's a good thing, because swords back then were just as awesome as swords today, if not better. Ditto for ancient tomes of lost mystic knowledge, legendary suits of armor, yadda yadda yadda. Guns don't work and play well with this sort of environment because we know that guns start a cycle of evolution that ends with something that is entirely incompatible with the basic design philosophy of D&D.