Why are some rocks more easily weathered than other rocks.

There a three types of rocks: Sedimentary, igneous, and metamorphic rocks.

Sedimentary rocks are made from small pieces of rock or dead plant material. They can be formed on land and are more exposed to the wind. This means that it is easily weathered.
Igneous rocks can also be formed on land, which means it can be easily weathered, too. 
However, metamorphic rocks are made in the earth's crust, or under the earth's surface. This makes it protected from any process of weathering.
So it really depends on where the rock is. If it is on the earth's surface, it is easily weathered. If it is in/under the earth's surface, it cannot be weathered.