Crossbows were extremely costly and difficult to make, not something a mere peasant could afford to buy or craft. The reason crossbows were anathematized by the pope and the crossbow makers and users threatened of excommunication was the deadly power of the weapon. There is several reports of crossbow bolts piercing through armors, shields and helms and some of the best crossbows were even repported to be able to kill a charging armoured horse in a single shot.
There are 2 main benefits of a crossbow over a longbow. The a crossbowman is cheaper to train than a longbowman. With longbows, it takes several years to train for effective use, as well as a lot of effort to be able to fight the draw force of a longbow. With crossbows, the user only had to fight the draw strength for a moment, until it locks, and often there were ways to get a mechanical advantage to draw the crossbow, such as ratcheting mechanisms or the popular foot-hole so you could also use your legs to draw the string.
Crossbows could be made very powerful, but most weren't. Most were cheap and easy, and that's what made them useful, you could quickly conscript peasants, spend a week or two arming them with crossbows, and have a decent ranged force. A longbow used by a trained individual would always outpace the crossbowmen, but you could always get more crossbowmen.
To note the power of a crossbow against the power of platemail, the arbalest, which was a heavy crossbow and very powerful, came into use 1200 CE and certainly after. Use of heavy platemail dominated warfare around the 1500 CE. Platemail only declined when firearms made platemail worthless. Crossbows were heavily used even during the 15th and 16th centuries when platemail was dominant, but it wasn't because they rendered platemail useless. If anything powerful crossbows created a niche for exceptionally powerful armor to balance it, a balance that wasn't broken until firearms. Crossbows and longbows could still penetrate platemail, but it was mostly because of a lucky or well aimed shot.
The real problem with DF crossbows is that they aren't crossbows, they act like crossbows that shoot warhammers. The projectiles have too much momentum, and when they deflect the blunt damage often breaks or jams bones through the armor. I have no problem with bolts penetrating armor occasionally, but they don't, they do damage by being really heavy. Hence the broken arrow mod.