Why Every Modern Companion Left Doctor Who
1. Varada Sethu - Belinda Chandra
At the end of The Reality War, Belinda Chandra finally got home (albeit home in a completely new reality) to continue co-parenting her daughter Poppy with ex-boyfriend Ritchie. Varada Sethu defended Belinda's ending, saying it was "beautifully tied up."
However, the reasons for Varada Sethu's exit are a bit messier. A leaked image of the Doctor and Belinda dancing in an apparent alternate ending for The Reality War implies the plan was for her to return in season 3.
It seems, therefore, that Varada Sethu's departure from Doctor Who appears to be an unfortunate by-product of Disney failing to order a third season of the RTD2 era. Obviously, a third season of Doctor Who doesn't appear to be happening soon, and so Varada Sethu was likely released from her contract to pursue other, more concrete, opportunities.