Because apparently, FreeStyle Games decided this was a better use of their time than making DJ Hero 3.
SiNG is not a typical video game. By some standards, SiNG might not be even considered a video game, but rather karaoke software built for Wii U.
At its base, there is a singing mode, in which up to two players can sing into the microphone and be evaluated based upon their performance in real time. This is standard stuff. The interesting part comes in when a theoretically unlimited amount of people can join in. While one person is the main performer, others can sing, clap, dance, use the Wii Remote as an instrument, use the GamePad as an audio mixer, or do whatever they want. Although there is such a diverse list of things the players are able to do, there doesn't seem to be much in-game feedback to what the external players are doing.
Other than the aforementioned audio mixing, it was also revealed that the GamePad can be used to manage playlists of songs during performances.
There is expected to be dozens of songs in the upcoming title, but these likely won't be consisting of your Neutral Milk Hotel songs or your obscure Billy Joel ballads from yesteryear. As the Nintendo E3 page on the game states, "A variety of song selections, from the 60s to current pop hits, makes it easy for players from any generation to put on a show."
SiNG releases on Wii U later this year.