Crossbows were more expensive than longbows and not (on average) more powerful, but massively less demanding on the users and massively more convenient to wield, just because of the way bows operate.
If you want to aim and fire a medieval longbow you have to stand up straight, in a very particular pose, aim with your whole body, sight by eye and instinct with a tool that has no aids to help you place the shot properly, draw through raw muscle power, and fire without taking too much time because the strain of holding the bow - if it's powerful enough to seriously threaten armored men and horses - is too great to hold steady for long. Then you do that again, and again, and again. In order to do this as effectively as the English longbowmen, you need years of conditioning and training, to such an extent that it reshapes your bones - and even so, your performance will start to drop in short order, and long before the battle's over you'll be exhausted. Firing large war-bows is nearly as strenuous as fighting in melee is.
If you want to aim and fire a heavy medieval crossbow, on the other hand, you wind it with a cranequin or a windlass - which is a relatively slow process, but not incredibly strenuous. You sight down the crossbow rifle-style, with the stock against your shoulder, which is much more intuitive and easier to learn than a bow. You can do so from pretty much any position; standing, kneeling, sitting, lying down, through a slit in a castle wall, between close-set crenelations, from behind a pavise shield. You may not have quite the same range and power as a master longbowman with a large yew bow does, and you certainly don't have the same rate of fire, but you're much safer, probably more accurate, and while your gear is a lot more expensive in cash terms you don't represent literal decades of training like that longbowman does. You can be a functional military asset in a few weeks and a master in a year or two, and if you die it doesn't take an entire generation to replace you.
That's the difference. Both bows and crossbows had very high-powered and very low-powered examples in the medieval period, but if you're raising and equipping an army then getting maximum performance out of a high-powered bow required extremely specific circumstances and a very long time horizon, while getting maximum performance out of a high-powered crossbow just requires money.