These types of volcanoes (those well inland) are especially powerful because of the type of magma involved. Continental magmas are thick and highly viscous. Thicker, more viscous (granitic) magmas tend to trap explosive gases more readily. Other types of volcanoes, like those in Hawaii and such, consist of basaltic magma. Basaltic magmas are a lot more fluid and so run out of a volcano, rather than blast. Volcanoes like Mount St. Helens, located at the boundary between continent and ocean, are a mixture of the two, and therefore intermediate in terms of their power and danger.
“mentos into a bottle of warm coke” kind of explosive power... on a planetary scale.