Acts 19:11 - 17
ESV - 11 And God was doing extraordinary miracles by the hands of Paul. 12 So that even handkerchiefs or aprons that had touched his skin were carried away to the sick, and their diseases left them and the evil spirits came out of them.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Because they were not doing it in faith, but as if magic style. It shows clearly that they did not have a close intimacy with Jesus, and were just religious. In fact, they said "in the name of Jesus (if not mistaken) of peter and paul."