Why is arsenic more chemically similar to phosphorus than is nitrogen?
Every element which is in the first place of the group, is pretty much different from the other elements of the same group. The reason is because the first element has ALWAYS the smallest radius, which leads to some changings also in group properties. Due to the small radius they are more stable than the others and they require more energy for any specific reaction. With some research you can see that also other  first elements of other groups such as Li,O,C etc.  differ from the other elements of that specific group.