Because it doesn't fit the fiction. Although crossbows were not difficult to learn (they still required a lot of practice of course), they were heavy and slow, especially the "heavy crossbow". While I can sort of see it with other casters (clerics and druids anyway), I can't really see it with wizards. Your typical wizard is not very strong, so using such a weapon would not be practical.
WotC butchered how crossbows should function anyway, so unless a group houserules them, D&D "crossbows" aren't historical crossbows.
So, it isn't a timeline issue for me, they just don't fit for wizards. And bring firearms of any kind into D&D is a no-no for me.
I'm not a huge fan of the pew-pew, either, but it fits fantasy for me better than wizards firing crossbows.