@fan_of_films said:

Michael Ontkean is not listed in the cast. Show won't be the same without his appearance. He was Cooper's closest collegue in the original series. I assume there will be some explanation why Truman is not in Twin Peaks. Perhaps they'll mention he retired to another town.

http://www.avclub.com/article/michael-ontkean-wont-return-twin-peaks-robert-fors-226549

I heard that the actor is retired from acting and was not interested in reprising the role.