This is because a dictionary attack is a type of password cracking attack that uses a list of common or previously leaked passwords to guess the credentials of a user. A dictionary attack becomes more effective because of global leakages of users' passwords, as the attacker can use the leaked passwords as a source for the dictionary. The source of this answer is the Cisco ENCOR v1.1 course, module 2, lesson 2.3: Implementing Wireless Security.