My guess is that it was expected that he would have some more prominence with at least one more line (or more). That giant white furry beast with a snorkel was built to be a character which could have had a longer participation as well.
Some of the stuff was shifted early one in production, and some more stuff was shifted in the reshoots. My guess bistan was nullified from the movie in the reshoots, because they gave him prominence in the feature videos to tout the practiccal effects they are using.
What probably happened is that Mickey Mouse signalled EA about the characters they would have, and EA ran with it. Later on plans shifted, but Mickey never told EA.
Ergo, we have Bistan, which is almost a non-character. Even Dr. Evazan and Ponda Baba would have been more significant for the fan base than him.
I will not even get started with Krennic and Death Troopers.