Why is magma with less silica less likely to cause explosive eruptions?

Because magma that is low in silica is less viscous (less thick) than silica rich magma. Silica rich magma is "gloopier" which means it is more difficult for dissolved gases to escape from the magma than thinner magmas. So when gases do escape the silica rich magma, they cause a bigger explosion, wheras silica poor magmas flow more readily