Lore wise it makes sense because The Farraki tribe joined with the Zandalari during the invasion of Pandaria. Additionally, they remained under the Zandalar empire through BFA, thus making the customization for Sandtrolls relevant to current lore and in-game standards. No longer are we just playing with Dark Spear Trolls but now a whole Troll nation with a variety of tribes to diverse from 
The new narrative they want is that your race is more open-ended. You are simply a troll who can come from multiple backgrounds based on your choice. This is why Exile’s Reach was added. To give your character a freedom in backstory.