Leia, Ewoks, and Tiger sharks.

In the real world, we've discovered in the last few decades that a skilled diver can actually *befriend* tiger sharks, along with the smaller lemon shark. We're talking belly rubs, getting jealous of other sharks, recognition after long periods apart, and earning enough trust to remove fish-hooks. These animals will also absolutely eat you if you act or move like prey; but if you stand your ground the right way and give them snacks, they'll categorise you as a peer- the evaluation is behavioural.

When Leia meets a suspicious, aggressive Wicket on Endor, she flat out refuses to be aggressive or frightened. She takes no guff from the Ewok, but shows affection and provides food. She doesn't emote fear or aggression, doesn't fight or flee, and throws in some food- Wicket's confused at first but can come to no other conclusion than to view Leia as "friend".

Chewbacca, on the other hand, is tempted by a fairly obvious chunk of bait, and falls into an animal trap along with Han, Luke, and the droids, who yell and thrash around in terror. "Prey" animals fall for snare traps, thrash around, and panic. Put 'em on the spit!

Han and Luke are the same species as Leia, but doing what prey animals do makes it very, very hard to change Ewok minds- these are highly instinct-driven predators. Sheer luck provides a cultural factor strong enough to cut through this.

The Imperials show aggression and alarm instantly. With no prophesised golden one and magic tricks to argue for them, their fate is sealed. On a deep, brain-stem level, the Ewoks cannot see them as fellow sapients, only as threats and food. Yub nub it is.