"Stronger" in what sense? The English regularly used longbow with over 100 pound draw weights. They had Yew which is an incredibly dense wood which allowed for the making of very strong bows.

As a bowyer, there are several reasons a composite bow is a disadvantage in northern Europe. The first would be materials, a composite bow is made of 2 long strips of horn, a strong wood core (Maple or mulberry), and sinew. The wood core and sinew aren't a major problem to get in Europe but horn is. They don't have the long, dense water buffalo horns that the East did, cattle and bison in Europe had smaller horns.

Second is the climate is almost too humid for sinew bows to exist, the sinew stretches out when the humidity is too high and the glues holding the composites together weakens. Look at places where sinewed composites were used: mainland china, Korea, the steppes of central asia, eastern europe, turkey, and egypt. They are all temperate to arid. The same can be seen in north America where sinew backed bows were made in the west and southwest while selfbows were used in the northeast.

Third would be cost. Horn composite bows are expensive and it can take a year or more for a bowyer to complete one. Compared to an english longbow that can be made in less than a day.