Why is alcohol dehydration mainly useful for producing ethers with simple primary alkyl groups?

Alcohol dehydration is mainly useful for producing ethers with simple primary alkyl groups because of competing chemical reactions. In the presence of an acid, the bimolecular dehydration of alcohols to form an ether must compete with unimolecular dehydration, which instead produces an alkene.1

Because this competing elimination reaction is highly favorable for more complex or branched structures, the bimolecular pathway only provides useful yields of ethers when using simple, primary alkyl groups, such as those found in dimethyl ether and diethyl ether. Consequently, while this method is the most economical approach for industrial production of these specific simple ethers, it lacks the versatility of other methods like the Williamson ether synthesis for creating a wider variety of ether structures.12