Question

why evaporation is called a surface phenomenon

Answer 1

Evaporation is a surface phenomenon because it occurs in the surface. For example when we put wet clothes for drying we spread them out so that the surface area gets increased. When the surface area increases the water will get enough space to get into vapour stage and thus evaporation happens easily.  This is why increase in surface area is a factor of evaporation. 
HOPE THIS HELPSSS .. 

Answer 2

the reason is that here in this evaporation process, the surface of the object is involved.When the temperature increases it breaks the bond of hydrogen and oxygen and as well know that the temperature is much on surface as in inner of it so due to breaking of the bond the molecules of the H2O gets evaporate from the surface......