Well in the case of the Sand Snakes, in particular, they weren't ever really meant to be include in the show originally. Dorne itself by and large was a last minute addition that D&D included because they saw that Oberyn was popular, they liked Indira Varma a lot and they needed Jaime away from King's Landing for the majority of the season for Cersei's plot to happen. We also know that they had larger plans for the Martells (Alexander Siddig was originally contracted to do 4 episodes in season 6 instead of 1) and they quickly cast those plans aside when reception turned as sour as it did.
Certain things like that happen all the time in productions where they craft specific elements to reflect what the showrunners believe may or may not be popular with audiences. Cersei being such a big element this late into the game instead of a newcomer is another example.