Ziva was dropped from NCIS when the actress playing her (Cote de Pablo) decided not to renew her contract.
I shared this question on Facebook, and the replies I got were:
She's making a movie called 33. It's about the miners that were trapped. Look on IMDB.
She moved on... She wanted something diferent for her life and career... Actors come and go... She is on movies now... Wish her well
Because Cote quit. Ziva isn't real
They pretty much some up Cote de Pablo really: she's a professional actress who has enjoyed a good run with NCIS, but it looks like it's now her time to move on.
You never know, she might come back for a special guest appearance some day!