Because Star Trek has a strong idealist humanistic message which means that every lifeform, even those with the most alien shape and morality, deep in their heart want peace and prosperity and can therefore be reasoned with.
This is why pretty much every conflict in Star Trek is really just a fantastic diplomatic dispute.
Star Wars on the other hand is full of irredeemly evil and powerful people (Palpatine, Jabba etc.) who are mostly motivated by their desire for power and who can only stopped by force.
Funny enough, the TNG movies are not much different from Star Wars movies with less diplomacy and more action.