Why does the Williamson ether synthesis work best with primary alkyl halides?

The Williamson ether synthesis works best with primary alkyl halides because the reaction relies on a substitution mechanism where an alkoxide ion acts as a nucleophile to displace the halide.1

For this substitution to be successful, the alkyl halide must be unhindered. If the alkyl halide is not primary—meaning it is secondary or tertiary—the steric hindrance around the carbon atom makes it difficult for the alkoxide to perform the substitution. In these cases, elimination reactions compete with the desired substitution, significantly reducing the yield of the ether.1