Why don't chicken and turkey sausages typically have sodium nitrate, but other meat sausages do?
Check this essay: Time-dependent depletion of nitrite in pork/beef and chicken meat products and its effect on nitrite intake estimation  

Check the Figure.1, while preserving meat in long days, the nitrite content in chicken sausage is much higher than pork/beef sausage.
For food safety, Nitrosamine where is transformed from nitrite is an important cancer factor. So there are only pork/beef(red meat) sausage in market but not for turkey/fish(white meat).