I guess they were smarter, but more naturally peaceful, and the newcomers either wiped them out and/or raped them out of existence.
Unlikely on both counts. They had bigger brains, but most of the increase in the back part of the skull associated with vision. They also had larger eyes, so I would guess they had much better vision than we do, especially night vision.
The lack of some genes associated with speech suggests they couldn't communicate as well as we can.
As far as peaceful goes, the modern humans who lived as the Neanderthals did (and who have a high level of Denisovan genes) are the highlanders of New Guinea. Their clans constantly fought ferociously and ate each other.