Look at what Daniels' attempt to protect Archer caused in "Shockwave" or Braxton's attempt to fix history in "Future's End". Look what happened after "Endgame" in the novelverse. Say Spock warns Starfleet about the Borg. Starfleet investigates a century earlier than the Hansons' did in the Prime universe and try something Kirk-like and reckless which brings the collective down on an unprepared Federation. Say he warns them about the Bajoran wormhole, which leads to an earlier Federation/Cardassian war. Say he warns Deneva about the parasites, and a century later a survivor's child starts experimenting with the omega and renders the entire alpha quadrant impassable to warp ships. I'm not saying Spock 100% shouldn't interfere, but I can see why he may choose not to.