Plot hole: When Picard is talking to Guinan inside the Nexus she tells him that he can leave and go anywhere anytime and Picard and Kirk go back to Veridian III just before the launch of the missile. Now why didn't Picard go back to when they first located Soran on the Armagosa observatory and detain him there? Or if he wanted to he could have gone back even further to Earth and saved his brother and nephew.
Suggested correction: This is a question, not a plot hole. Characters behaving differently than you would doesn't constitute a plot hole, and there's nothing contradicting the plot. The short answer is because of the Temporal Prime Directive. Yes, there have been several Star Trek episodes and movies dealing with time travel, but Picard knows it's best not to change the past too much. Plus, going back even further in time doesn't guarantee he'll be able to stop Soran.