But we are not under the old covenant any more.
And if you incorrectly think we are, I will follow your illogic: then this command would apply within the country of Israel which - for better or worse - has not yet conquered the USA.
Perhaps not, but Jesus sure seemed to use the memory of the sulferous rain on S&G to make a point.
Matthew 10:14-15
14 And if anyone will not welcome you or heed your words, shake the dust off your feet when you leave that home or town.
15 Truly I tell you, it will be more bearable for Sodom and Gomorrah on the day of judgment than for that town.