No clue what mamdani is like, but it seems like NYC had little to no choice...which is a bit disappointing.
Approval voting has all the benefits ranked choice claims to have, but without the complexity - it's trivial for people to understand: https://en.wikipedia.org/wiki/Approval_voting
Mamdani won the primary for the democrats over Cuomo, but Cuomo decided to try and do an independent run to further challenge him.
Typically, the Republican candidate would have no chance in a city like NYC. This was the case here as well, but Cuomo calculated that with the backing of establishment Democrats AND the backing of Republicans/conservatives, he'd be able to defeat Mamdani. The Republican candidate did not agree to drop out, however. In the end it didn't matter though because Zohran Mamdani won by a larger margin than Cuomo and the Republican combined
In a typical election, the main election is the primary (which happened back in June). The Democrat nominee is pretty much guaranteed to win so the general is almost a formality. This general election was actually more contested than is typical
tl;dr: his main opponent was establishment democrats
13% is not narrow
The entire establishment marshaled what forces it could to stop mamdani's momentum. Couple this with the fact that there are (unfortunately) many people out there who would rather elect accused sex offenders than risk the chance that somebody marginally aligned with a word and ideology they don't actually understand (socialism) would be elected, or more likely, and worse, people are just racist and/or islamophobic and would sooner elect a man who would grope their daughter than a man who, god forbid, has a different religion than them.