Answer
The factor of the introduced species had not been a factor on generations of selection prior to its arrival.
Work Step by Step
Because of this, prior generations of other species in the area will have no adaptations associated with this introduced species. Any predation will be unspecialized, if present at all, and because pathogens and parasites tend to be so heavily specialized they are even less likely to already exist in the new environment. Selection pressure depends on existing factors over many generations, something that was not possible before the introduction.
