The sales numbers for DJ Hero are in, and they are disappointing at best. 122,300 copies of the game were sold across all platforms. That's very low for a music game, especially one that was as heavily promoted as DJ Hero. Ars Technica attempted to explain the low sales, and fans of co-op will likely appreciate this particular reason:
Rhythm games work best with crowds, and DJ Hero is a solitary experience. That's not a knock against it; playing the game is great fun. It's just hard to share with others, compared to the more popular Rock Band and Guitar Hero franchises.
In other words, the lack of a compelling co-op mode may have been a factor in the game's lack of sales. Music games seem to work best in a party setting, and DJ Hero just doesn't work due to the single-player focus. Other reasons are listed in the article, but it just goes to show you that co-op is a major selling point for any game, especially one in a genre known for excellent co-op.
Source: Arstechnica.com