Major concentration around East Coast U.S. and Europe
Location keeps the forests cool year round, with an average temperature of 50 degrees. The average for precipitation is only 30-60" a year, so there's enough water for some plants to grow in Spring and Summer.
Deciduous forests aren't nearly as hot or wet as tropical forests, so insects are much less common. They are also self sustaining, as leaves decay in the fall and fertilize the soil so more plant life can grow.