Primary alcohols preferentially undergo SN2 reactions because their central carbon atom is less sterically hindered, allowing the nucleophile to attack directly and displace the leaving group in one concerted step.
Secondary and tertiary alcohols, on the other hand, are more hindered, making it difficult for nucleophiles to approach. Therefore, they tend to undergo SN1 reactions where the alcohol leaves first, forming a more stable carbocation intermediate, and then the nucleophile attacks in a separate step.