Maybe the togetherness is a commonality. But even at galas like debutante balls, the individual debutantes are still celebrated in a way that prom goers aren't. A lot of the balls have quite exclusive guest lists, so even going is a huge thing. A prom? Not exclusive at all.

I also disagree that it doesn't matter for the "specialness". A better comparison for a quinceanera is a special sixteen party. There is a communal aspect, but the difference is that all the people there at a quinceanera or sweet sixteen are there to celebrate YOU. At a prom, everyone is there to celebrate themselves. They're both big fancy gatherings, but it's undoubtedly different to have a whole party in your honor. As for the private parties leading up to prom, they can have those any time. They don't need prom as a reason.