The reason could be that all collisions do not lead to the
formation of products The collisions in which molecules collide with
sufficient kinetic energy (called threshold energy*) and proper
orientation, so as to facilitate breaking of bonds between reacting
species and formation of new bonds to form products are called as
effective collisions For 
example, 
formation 
of
methanol from bromoethane depends
upon the orientation of reactant
molecules 
as 
shown 
in
Fig 3