Folding bikes use smaller wheels. The small wheels allow for compact storage, which is wonderful. However the small wheels minimize the distance to the ground. Therefore pedal strikes on the ground become a risk. A pedal strike can cause a crash, hence it is important to minimize the risk. 

How does Downtube minimize pedal strikes? First we use high bottom bracket height, the higher BB minimizes the risk of a pedal strike. We also use shorter cranks ( 170mm ) to minimize the distance to the ground. Lastly we use narrow pedals to minimize risks.