Trafficking refers to people who are generally not slaves in their home country but are lured or transported to a foreign country to be exploited. It’s a distinct phenomenon, so I don’t see why it should be lumped under slavery.

How is it distinct? They're enslaved abroad! That doesn't make it any less slavery than if they were enslaved in their home countries.

The concern is because it's almost impossible to have slavery in a place where it's (de facto) illegal without some trafficking.

Trafficking is what enable slavers to stay hidden.

Moving slaves around, during which the authorities may notice, is a better hiding strategy than keeping them in one location? Are you quite sure?

Then keeping them around their home and families? I'm quite sure.

Two things can be bad and we're allowed to complain about both.

But either way trafficking is typically shorthand for slavery in practise. The thing with trafficking specifically is that it is more likely to go somewhere that it can be spotted. Airports have signs in the bathrooms saying "if you're being held against your will try to get to this part of the airport" because it's the trafficking piece of the slavery/trafficking combo that takes the victim into the airport in the first place. Being trained to spot trafficking in jobs like TSA agents is thus very useful regardless of the slavery situation.

But it's not two things. One is a subset of the other.

It's as absurd as creating a special term for whipping slaves using sticks of a certain color.

I'm really not seeing what you're complaining about unless you have some "all lives matter"-esque point that you're trying to make. Nobody's communication is being impeded by saying "let's stop sex trafficking" instead of "let's stop sexual slavery" and nobody's behaviour is going to be different on hearing those two sentences and nobody is going to pass different laws or behave differently in any way. I hadn't even thought of those two as separate previously but even taking the effort to mentally separate them to argue with you, I don't feel enlightened or challenged to act differently. There's nobody out there saying "Trafficking is bad but slavery is a-OK!". I really don't know where you're going with this.

Why did we feel the need to isolate a minor aspect of slavery, teach everyone the new term, and then arouse outrage against it, instead of simply organizing against "slavery", which had already been considered wrong for generations?