One of the great advantages of the cross bow is that it takes relatively little training for someone to get procient. Also, you don't need the physical strength a war bow shooter has to have. They can stay loaded and spanned for a long time, and you don't need as much room as a long bow.
Making one is much more complex and difficult, though, so they would have been far more expensive than a yew bow. Less material needed for the bolts vs. a war arrow, too.
Dane