That question could be asked for every time travel episode in the history of Trek, why was the Relativity or other time ships never there to correct the problem in the timeline?
Cause up until the episode Relativity, it didn't exist!
Or perhaps the changes were meant to happen, and they just sat back quietly and let matters unfold. What do they call it, pre-destination paradox or some such?
Kim: I'm detecting some weird technobabble.
Janeway: A possibly dangerous anomaly that we know nothing about?
Kim: Yeah. I suppose we should steer clear.
Janeway: Ha! Good one, Ensign. Take us in, Mr. Paris.