If there's a legit reason, it's probably that Picard didn't want to get into a double-Picard situation by going back further in time. (That was awkward enough in "Time Squared.") But it's confusing enough already that he could place himself back into the timeline at a point before the nexus had even arrived. How he was able to accomplish that is anyone's guess, so it's not clear how he would've been able to place himself farther back to prevent other things from happening, too.Well, yeah, or at best just hang suspended in space in a space suit until the Nexus flew into you.
But while that's a flaw, it's not the biggest. Picard specifically chooses a point in time to return to that not only makes no logical sense, he also needs the help of another person (re: Kirk) to help him stop Soran. But if Picard had chosen, say, the day before to go back, not only would he not have needed Kirk (and not caused Kirk to die in the process), he could have just told his command staff on the Enterprise what had happened (since the exact same time displacement had just happened to Picard in "All Good Things..." and his crew would have understood), and then just arrest Soran at the Amargosa observatory. But apparently logic doesn't make for a movie script.
Putting the 2 captains together was nice, but it all could've been done way better.