NYC is something like 20% Muslims. They got out the vote and voted for him close to 100%.

There also are a lot of cucks and commies in NYC. Mamdani also got most of their votes.

There was a mainstream media hype cycle promoting him, similar to what they did to Obama. Around 1-2 months before the Democratic mayor primary, there were lots of "news" stories on "Check out this interesting longshot candidate running as a Democrat." As he got more news coverage, he steadily rose in the polls, became the frontrunner, and then won. Once he won the primary, he was a near-lock to win the general election.

Some people say Cuomo ran to lose on purpose. Cuomo didn't get enough votes to win, but he did get enough votes to prevent a better anti-Mamdani candidate from getting traction.

The Republicans didn't even try, letting Sliwa get the Republican nomination unopposed.