And they added them for context with the story. Hunt vampires with crossbow? Yeah i have seen that before. Which is why you should be able to use one as Artificer in DA:I aswell. Actually having specific weapons for specializations would be awesome.
Yes, they had to have context to add them because the Elder Scrolls had one spot before Dawnguard and then removed them, they were in Morrowind and then poofed from existence. We had crossbows in Dragon Age:Origins, and then in DA 2 they somehow got limited specifically to Varric so he could be a special snowflake.
Its the same problem as breaking down the weapon groups based on class, I felt so very underwhelmed when my Warrior was forced into "Column A" and "Column B" for his weapon choices. A knight in most cultures learned how to use: the longsword, mace, ax, lance, bow, crossbow, most two handed weapons, and the shield. In some he would learn traditional fighting stances that used a buckler, a cloak or a secondary weapon for dueling purposes. He however wasn't strictly limited because knights in the crusades were from all walks of life and got trained in a superfluous amount of weapons. Even when you do the spread across to the Asian culture: The Samurai learned the Katana, Wakizashi, Tanto, Spear and the Bow. Dual wielding in that culture is a manifestation of the teachings of Musashi, but it did become wider spread after his death.
As for Rogues, if you were to put them in a historical perspective you'd probably have more of a peasant, bandit or deposed noble as a rogue in which case you could have the following possibilities with a peasant in mind: Crossbow, farming implements such as a sickle or an ax, a spear, a bow (possibly), sometimes the sword and probably at least a basic knowledge of a shield. The deposed noble has a few historical examples but tends to be in historical fictions or fantasy, their weapon knowledge would have roughly the same berth as a knight.
TL:DR We had crossbows before and there's plenty of spots where it shouldn't be a weapon limited by class, even if all classes don't make the best use of them.