@AvnishKabaj Well ... Don't laugh but $185$ :'( ... I have lost around 50 marks purely because of silly shit !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
As far as I remember(from Peter Sykes) here is the explanation-SN1 reactions have no role of nucleophile in rate limiting step(hence the term SN1).Stronger nucleophiles tend to generally participate in rate limiting step and hence SN2 is favoured by stronger nucleophiles.
As to why they tend to participate in rate limiting step-They have high charge density and hence do not have to wait for C+ formation and can compel reaction to proceed in a single concerted step