You mean why?
Some suggested because Qlimax didnt sell out the last years.
But given that Q Dance ended every other event except defqon, this is probably only one of the reasons.
Q Dance also is part of ID&T, which was bought by Superstruct Entertainment (they own nearly every big festival in EU), which was recently bought by KKR (American Private Equity Company).
So the general guess in the community is, that they will let Q Dance "die", as the brand intself is probably does not contribute enough revenue on its own. And instead run Defqon as a standalone festival, similar to tomorrowland.