Strength of nucleophiles among SN1 and SN2 reactions
This seems to be true only because SN1 reactions prefer polar protic solvents which stabilize both the leaving group and the carbocation. But such solvents also surround strong nucleophile and hinder it from the carbocation.
Weak nucleophiles on the other hand don't interact with protic solvents as much and thus stand a better chance of getting close to carbocation.