"And that servant who knew his master’s will but did not get ready or act according to his will, will receive a severe beating. But the one who did not know, and did what deserved a beating, will receive a light beating." (Lk 12:47-48)Sodom may've gravely sinned against God, but done it without knowledge and in accordance with their human nature. The residents of Capernaum would not have that excuse though, which is why their fate would be worse than that of Sodom and Gomorrah.
Capernaum would not be the only city to suffer this fate - elsewhere in His ministry, Jesus sent 72 of His disciples out ahead of Him to work in His harvest field. If a city accepted the disciples in their ministry, the peace Christ gave the disciples would rest on the household. But if a town rejected them, the peace that God had given them would return to them. Christ said about such towns:
The Holy Bible : English standard version. Wheaton: Standard Bible Society.