See, Cthullu Mar-vell tries to kill Thanos, but it summons Death into their universe, who then kills all the cthullu people (can't escape Death twice). Death then leaves Thanos stranded in the cancerverse, who's all kinds of pissed. Star Lord and Nova stay behind to make sure that Thanos can't get back to our universe while the pocket gate closes with the remaining cosmic cube power, and to make sure they face Death screaming. And everyone thinks they're dead now, since there is no way to get in contact with them since the rift has been closed, and the universe is supposed to now collapse into itself since the cancer gods are dead. And they have a statue dedicated to them and everything, hella sad.
On top of that, Phylla is dead, Drax is dead, and Warlock is dead. And the Nova force is all gone.
I mean, Quill did have the cosmic cube, albeit damaged, so there is a chance of pulling them out of there in a year or two, but still, it hurts.