Because those that often bring it up do so to try and get things for free such as reparations and what not. That and they haven't been affected by slavery for generations. Instead how about look at where you would be if it weren't for the white man bringing you here, slaves or not, you're way better off in this country than you would have been if you stayed if Africa.
Also one of the most annoying aspects is that they always seem to either ignore/don't know the fact that they were sold to whites by their own black African countrymen
Oh and the fact that they lump all white or white looking people together, blaming all whites for slavery when reality is the majority of whites never fucking owned them. In fact if it wasn't for a White man they'd still be fucking slaves. Show some fucking gratitude already