From the ACF FAQ:

http://acf.org/Q&A.htm#11

"There are a series of viruses that attack the blight fungus in Europe,
rendering it incapable of causing severe cankers on European chestnut.
This seems to indicate that these hypoviruses have made blight a
relatively minor disease on European chestnut. In the United States,
despite extensive efforts over the past 30 years, the same viruses have
not resulted in a significant decrease in blight severity on American
chestnut, especially in forested settings. However, pure stands of
American chestnut located in orchard settings have been protected from
blight to the extent that they continue to live and produce nuts.
Unfortunately, those trees are too disfigured by blight cankers to
produce sound timber. Additionally, the disfiguring cankers quite likely
would make those American chestnut incapable of competing with other
tree species in forest settings.

The reasons are not entirely clear why hypoviruses give good control of
blight in Europe but not the United States. It is known that there are
many fewer strains of the blight fungus in Europe than in the United
States and that hypoviruses spread much more easily within strains than
between strains. Thus, cankers on European chestnut initiated by
virulent strains of the blight fungus are much more likely to become
infected with hypoviruses. Additionally, European chestnut is slightly
less susceptible to blight than American chestnut, which would make
cankers persist longer on European
chestnut, increasing chances for hypoviruses to invade them.
Finally, European chestnut trees tend to grow in fairly pure stands,
without competition from other tree species. Competition may weaken
American chestnut trees, increasing blight severity to the point where
trees die before hypoviruses can act."